Pete says it's not a part of the creative process when making the films/stories. Ronnie later reaffirms that statement as whole. During the 2015 D23 Expo during "Pixar Secrets Reveal" panel (which also featured Director Dan Scanlon, Senior Producer Darla K. Anderson, Director Mark Andrews, Senior Development Executive Mary Coleman, and Co-Director Ronnie del Carmen) Mark Andrews rejected the theory.
"It's not true - come on!" - Mark Andrews
"Do you know what kinds of meetings we’d have to have to make sure all our movies line up?!" - Ronnie del Carmen
The same publishers of those videos are the same ones that published this article which those quotes stem from,
10 Secrets We Learned from the Pixar Secrets Revealed Panel.
Disney did purchase Pixar but some of the conditions was that Pixar would remain as its own entity, identity and culture.
Again, Disney never made such claims. No where in the articles and videos does Disney state all of the films are set in same universe.
One may interpret that the theory exist. Sure, no one is taking that away from anyone, but Disney and Pixar don't acknowledge it.