Pure Python code always needed mutexes for thread safety with or without ol' GIL. I thought the difficulty with removing the GIL instead had to do with C extensions that rely on it.

This is accurate and the parent commenter here seems to be echoing a common misconception. Either they are confused or they need to elaborate more to demonstrate that they have a valid complaint.

For instance, this would have been a valid complaint:

"Users who don't need free threading will now suffer a performance penalty for their single-threaded code."

That is true. But if you are currently using multiple threads, code that was correct before will still be correct in the free threaded build, and code that was incorrect before will still be incorrect.