It appears the Lord Buddha criticized natthikavādaṃ (moral nihilism) because it is doctrine based on grasping (upadana). In other words, the Lord Buddha appeared to not criticize natthikavādaṃ because of disbelief in an afterlife.
In Buddhism, the word "grasping" is "upadana", which means to "take something as one's own". Due to views of "self" or "ego", selfishness arises. Selfishness leads to moral nihilism -- the mind is grasping at dirty animal lust as "me" and "mine" and "I". This dirty grasping at dirty things, leading to harm & trauma of everyone involved, is natthikavādaṃ.
For minds out of touch with reality, some suttas about "grasping" and "selfishness" are as follows:
Now, craving is dependent on feeling, seeking is dependent on craving, acquisition is dependent on seeking, ascertainment is
dependent on acquisition, desire and passion is dependent on
ascertainment, attachment is dependent on desire and passion,
possessiveness is dependent on attachment, stinginess is dependent on
possessiveness, defensiveness is dependent on stinginess, and because
of defensiveness, dependent on defensiveness, various evil, unskillful
phenomena come into play: the taking up of sticks and knives;
conflicts, quarrels, and disputes; accusations, divisive speech, and
But viewing consciousness as "cognition" is not "grasping". This is why the Lord Buddha said the reality of consciousness is that is cognises.
The question asked makes no sense because the question seems to irrationally believe the idea of a "thing" is the arising of "self" or "grasping"; such as to view a computer as a "computer" is "grasping".
The Lord Buddha did not to teach to abandon the idea of "things". What the Buddha taught to abandon is described below:
16. "But, monks, there is here a well-instructed noble disciple who has regard for Noble Ones, who knows their teaching and is well
trained in it; who has regard for men of worth, who knows their
teaching and is well trained in it: he does not consider corporeality
in this way: 'This is mine, this I am, this is my self'; he does not
consider feeling... perception... mental formations in this way: 'This
is mine, this I am, this is my self'; and what is seen, heard, sensed,
and thought; what is encountered, sought, pursued in mind, this also
he does not consider in this way: 'This is mine, this I am, this is my
self'; and also this ground for views (holding): 'The universe is the
Self. That I shall be after death; permanent, stable, eternal,
immutable, eternally the same shall I abide in that very condition' —
that (view), too, he does not consider thus: 'This is mine, this I am,
this is my self.'
In conclusion, insisting on consciousness as an objective real thing does not inherently lead to unethical behavior (moral nihilism) just as the Buddha warned. Only insisting that consciousness is "self" or "me" or "my" inherently leads to unethical behavior (moral nihilism) just as the Buddha warned.