Nobody is disputing that Mahabharata has layers that were added in the post-Mauryan period
The issue is with the insinuation that it is "largely" a post-Mauryan work, which academics tacitly encourage
To the extent that most ppl outside India describe MB as a "2000 yr old text"

Though I have my issues with the RW reaction to the Thapar clip, I see in part the reasons for the rage.
There is a certain lack of trust. People don't trust her intentions.
Before we know it, MB will be described as a Gupta-age work even.

Distrust of indologists is well founded, as we've seen this play out before
When everything from vegetarianism to asceticism to austerity to "ethics" has been attributed to "shramaNa" religions
As though the old vedic religion was a heartless religion of ritual and little else

This is not to say the shramaNa religions had no impact on the brahminical religion and its priorities
But the issue arises when you overstate the influence of one on the other, and discount the influences in the opposite direction

It becomes particularly dicey when texts don't have dates, and we lack inscriptions / manuscripts from the period.
So indologists are free to decide which Upanishad is post-buddhist and which one isn't, as per their ideological agendas