I said they were, and are, free to talk at the Oxford Union. Just as people are free to protest. It's not a University, it's a members club. But not to teach whatever they simply believe. You really don't want a homeopath teaching medical students, a holocaust denier teaching history, Donald Trump teaching biomedicine or Andrew Tate teaching women's studies.
Those certain well-defined exceptions you mention aren't well defined. Particularly in Universities with new legislation which claims to protect lawful free speech but not what the government considers unlawful free speech. These aren't clear or fixed, as indicated by moves to criminalise groups like Just Stop Oil.
Universities are also bound by the equalities act and are supposed to protect students from harassment & unlawful discrimination.
I've not got a problem with the Oxford Union inviting who they like, beyond it being an elitist members club that delights in the publicity of manufactured controversy.
I also don't think Stock should have been subjected to the levels of bullying and harassment that led to her feeling unable to return to work. Her University could have done much more.
But let's be clear, she wasn't invited to the OU to talk about aesthetics, her area of academic expertise. She was invited to talk about her gender critical views knowing the publicity they would get from the protests.