It's nothing to do with personal boundaries. It's about intellectual boundaries and keeping the discussion honest. When the focus shifts to calling perceptions, motives, character into question, it's usually the end of meaningful discussion. It's ad hominem logical breakdown.

Related to the topic of this thread.... I faced similar problems when I tried to explain to therapists that the previous therapist had s**t the bed. Many of them wanted to immediately locate the source of the problem within me. They could not face the thought of therapy itself becoming a festering pustule of dysfunction. It showed me a profession afraid of its own pathology.

It's clear to me a core assumption is that the client is not a credible witness to their own experience.

So, yea, therapists as a group seem to have trouble dealing with being wrong.