If the Buddha said that ultimate reality is really non-self (anatman), or empty of inherent existence (sunya), then why did he bother to talk about human beings and other “provisional” things?
In the Buddhist context non self has a special meaning which is not worthy of identifying as self because it is not always pleasant, there is not permanent core, there is not internal or external (God) controller.
Also there is conventional and realities. As social conventions you will refer to someone as an entity. This is for the sake of communication.
A seeker came to Buddha and started to talk about his studies. He tried to seem knowledgeable, worthy a discussion with Buddha. He wanted to get appreciation. He wanted his attainments to be acknowledged.
Buddha listened to him, gently smiling.
Eventually the seeker had finished talking. He thought Buddha would answer then, but Buddha kept silence.
What would you do in place of that seeker?
Thoughts jammed in his head. He didn't know what else to say, every attempt to continue the talk he could imagine seemed stupid.
What happened next?
After some time the seeker respectfully bowed and silently left. What did he understand?
Did he understand what Buddha meant by his silence? Maybe he just pretended to have understood, to avoid looking stupid?
To answer this, we need to be able to see beyond words, beyond formal ideas.
That ability to see beyond words was the aim of Buddha's teaching about the ultimate reality.
It was not just to change the vocabulary.
Would that be important, whether that seeker talked about shunya or about human beings?
Do you understand?
Talking about shunya and about human beings for Buddha was not different.
His words were always relative truths. The ultimate truth was never in the words, and never could be.
That's why Buddha spoke about human beings.
Subsequently, many Buddhists misunderstood what is absolute truth and what is conditional truth. Nagarjuna explained that, but even today many practitioners, even many Buddhist teachers have distorted views on that. Be very careful. It's so easy to get incorrect understanding and cling to it thinking that you "know".
One reason given in the suttas is that he did it by request, and out of compassion.
As the Blessed One reflected thus, his mind inclined to dwelling at ease, not to teaching the Dhamma.
Then Brahma Sahampati [etc...]
Then the Blessed One, having understood Brahma's invitation, out of compassion for beings, surveyed the world with the eye of an Awakened One.
The Buddha's not viewing the world as 'self' wouldn't stop him from having compassion.
Compassion is one of the four recommended attitudes to have towards others.
Also, views such as,
There is nothing given, nothing offered, nothing sacrificed. There is no fruit or result of good or bad actions. There is no this world, no next world, no mother, no father, no spontaneously reborn beings; no contemplatives or brahmans who, faring rightly & practicing rightly, proclaim this world & the next after having directly known & realized it for themselves.
... are defined as "wrong view"; I guess that would be some kind of "nihilism".
Your question mean nothing in present, in the past, and in the future.
But buddha mean present thing is nothing in the past, and present thing is nothing in the future.
Ultimate realities of human being are causes and effects.
Nibbana ultimate reality is perfect finished of causes and effects.
Anatta is uncontrollable to being or nothing. There are just causes arising to make effects. And effects vanishing because of causes vanishing. No self, no atta.
Ultimate realities all is anatta. Ultimate realities of human being is present, past (present of it's past) and future (present of it's future). They are present because of their causes arising. They are past because of their causes vanishing. They are future because of their causes still possible to arise.
Nibbana ultimate reality is no time because it has not any cause, so no any effect, too.
The Buddha's reality is relational - non-Self is the inferred (observational evidence at enlightenment MN27) understanding that if 'things' are only ever observed as impermanent, they cannot be said to be observed to have a permanent core (or shell, or any other observed aspect). Things are not provisional, simply subject to conditions.
'You' and 'I' are not provisional in the Buddha's words, rather the concepts are subject to a mother and father for the conditions of birth, food and water for the conditions of life, and some other conditions for death (say a spear in the eye, or lack of food). This is not saying anything more than we already see - things are not observed to exist independently in and of themselves, independent of others.
This is not a nihilistic existence, but experience and values are derived from relations, not things in and of themselves.. all experience being observed as impermanent and lacking any and all notions of Self, universal values (Dhamma) are therefore rationally derivable (subject to the continued validity of the observation of impermanence).
(https://en.wikipedia.org/wiki/Relational_quantum_mechanics for modern evidence as to the effectiveness of the world view).. from a relational QM point of view, properties arise only with observation and interaction (not without observation as that would be unverifiable - both unscientific and un-Buddhist), with individual quanta empty of independent properties. In other words - empty jenga blocks don't build concrete, well delineated Selves - 'I' is a useful everyday notion, but no thing certain or permanently delineated within the structure of experience. The notion of Self is frame dependent - dependent on observation, subjective.
The objective truths of the world are those of impermanence, non-Self and unsatisfactoriness with such a 'build' for psychological systems (the last point being the first step to building the Buddha's world view - 'limited resources, unlimited wants', but with evidence beforehand)