I think this practice applies better to discussions of style or technique rather than actual mistakes. I agree with you that mistakes should be blameless, talked about frankly and without shame. Style disagreements, however, can be tricker because they're based on matters of personal preference and are tied more closely to our identities as developers.
For example, if I'm looking at the existing codebase and one of my teammates disagrees with the way I extracted a method, if she says "why did you do it this way?" this frames the question as an interrogation of my personal style choices. Asking "why do we do it this way?" acknowledges that this is a design decision we've adopted as a team (since my code has made it into production) but could perhaps be done better. That means that the question is coming from a shared responsibility to make the codebase better for everyone, rather than a personal distaste for someone else's style.
Of course, ideally, we all keep our ego out of our code and our judgment out of our questions, but as we live in an imperfect world, this seems like a good workaround to me. :)
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