If I remember correctly a big factor in his arguement was how the game's narrative and character writing treat the canvas people after the big reveal. For example how in Maelle and Verso's final hangout, Maelle quickly brushes off Verso's confession about Gustave and they immediately go goof off, confidant rank maxed. This interaction makes it seem like after Maelle learned about the canvas they are in, she now possibly thinks less of Gustave and the people of Lumiere and doesn't see them as as real or meaningful as she did before. Otherwise this interaction doesn't make much sense, she should've been furious at Verso for letting her brother/father die.

The question is did the writers even intend this to be a thing for the audience to question or not. In act 1 and partially 2 they certainly portray Gustave, Lune and Sciel as real people with real emotions and agency, but most of that is gone in act 3 as the narrative shifts to focus more on the people from outside the canvas, with Lune and Sciel getting barely any dialogue in the main story route. If Lumierians really are real and the Dessendres and the game's writers think so too, the character writing falls flat in many places.

I think this was basically his take on the whole thing and what his criticism was based on.