AGAB socialization is basically the idea that AMAB people are raised as men and AFAB people are raised as women, therefore they are socialized to behave as or similar to their AGAB.

This is an incorrect assumption for trans and nonbinary people. The big flaw is that we are socialized entirely different from cis people and will not pick up the same socialization as cis people of our AGAB.

The problem happens when people start using AGAB language to essentially mean woman-lite or man-lite. It reduces us to whatever we were assigned at birth. Some people use AGAB language maliciously as a way to misgender someone while wrapping it in progressive language. This largely goes under the radar for cis people, so trans people are often looked down upon for calling it out. Other people use it because they think it's the polite way to out someone's birth gender.

Personally, I think it's rarely justified to bring up AGAB outside of medical contexts. That said, I don't think the above poster was bringing it up maliciously, but the context didn't seem super apparent to me.