The following sutta quotes from Gihi Sutta and Sundarika Sutta show that a person of any caste, social class or background can become an Arahat. The Therigatha has numerous examples of women who became Arahats.
From the Gihi Sutta (AN 5.179):
In the same way,
wherever one is born
among human beings —
noble warriors, brahmans,
outcastes, or scavengers —
if one is tame,
with good practices,
consummate in virtue,
a speaker of truth,
with conscience at heart,
one who's abandoned birth & death,
completed the holy life
put down the burden,
done the task
gone beyond all dhammas,
through lack of clinging
offerings to this spotless field
bear an abundance of fruit.
From the Sundarika Sutta (SN 7.9):
Then Sundarika the brahmin went up to the Buddha, and said to him:
“Sir, in what caste were you born?”
“Don’t ask about birth, ask about conduct.
For any wood can surely generate fire.
A steadfast sage, even though from a low class family,
is a thoroughbred checked by conscience.
However, is it true that only a brahmana or kshatriya can become a Buddha?
I think it's the other way round. The Mahāpadāna Sutta quoted by that paper simply implies that the bodhisattvas (the beings intent on awakening) tend to have a karmic background that usually results in favourable birth destinations, rather than stating that the bodhisattvas can NEVER be of lower castes.
For e.g. professional basketball players tend to be tall persons, but it does not mean that ONLY tall persons can become professional basketball players.
We can relate from the Tamonata Sutta quote below that the bodhisattvas tend to be persons in light who are headed for light, by virtue of their good conduct and mindset:
"And how is one the type of person in darkness who is headed for
light? There is the case where a person is born into a lower class
family — the family of a scavenger, a hunter, a basket-weaver, a
wheelwright, or a sweeper — a family that is poor, with little food or
drink, living in hardship, where food & clothing are hard to come by.
And he is ugly, misshapen, stunted, & sickly: half-blind or deformed
or lame or crippled. He doesn't receive any [gifts of] food, drink,
clothing, or vehicles; garlands, perfumes, or ointments; bedding,
shelter, or lamps. He engages in good bodily conduct, good verbal
conduct, & good mental conduct. Having engaged in good bodily conduct,
good verbal conduct, & good mental conduct, he — on the break-up of
the body, after death — reappears in the good destination, the
heavenly world. This is the type of person in darkness who is headed
for light. .....
"And how is one the type of person in light who is headed for light?
There is the case where a person is born into an upper class family —
a noble warrior family, a priestly family, a prosperous householder
family — a family that is rich, with much wealth, with many
possessions, with a great deal of money, a great many accoutrements of
wealth, a great many commodities. And he is well-built, handsome,
extremely inspiring, endowed with a lotus-like complexion. He receives
[gifts of] food, drink, clothing, & vehicles; garlands, perfumes, &
ointments; bedding, shelter, & lamps. He engages in good bodily
conduct, good verbal conduct, & good mental conduct. Having engaged in
good bodily conduct, good verbal conduct, & good mental conduct, he —
on the break-up of the body, after death — reappears in the good
destination, the heavenly world. This is the type of person in light
who is headed for light.
Additional quote from "Buddhism in a Nutshell" by Ven. Narada Mahathera, to show that people from not-so-favourable backgrounds joined the monastic order, and became arahats or otherwise important members:
It was the Buddha who first attempted to abolish slavery and
vehemently protested against the degrading caste system which was
firmly rooted in the soil of India. In the Word of the Buddha it is
not by mere birth one becomes an outcast or a noble, but by one's
actions. Caste or color does not preclude one from becoming a Buddhist
or from entering the Order. Fishermen, scavengers, courtesans,
together with warriors and Brahmans, were freely admitted to the Order
and enjoyed equal privileges and were also given positions of rank.
Upali, the barber, for instance, was made in preference to all other
the chief in matters pertaining to Vinaya discipline. The timid
Sunita, the scavenger, who attained Arhatship was admitted by the
Buddha Himself into the Order. Angulimala, the robber and criminal,
was converted to a compassionate saint. The fierce Alavaka sought
refuge in the Buddha and became a saint. The courtesan Ambapali
entered the Order and attained Arhatship. Such instances could easily
be multiplied from the Tipitaka to show that the portals of Buddhism
were wide open to all, irrespective of caste, color or rank.