Maybe it's the same as with girl guides.
They changed their policy without really telling anyone.
The individual groups have not been explicitly told or given guidance afaik.
Guide leaders in general are women who really care about girls and volunteer their time and energy, including loads of paperwork, chasing payments, responding to random questions about all sorts from parents in their spare time etc.
They don't know about this. I imagine most would be ???!!! It's not what they signed up to. Which was a well established charity with an exemption around being female only due to girls particular challenges in society.
If the kickboxing people are like when my kids did judo. It's similar to guides. Local volunteers who give their time etc.
The tweet doesn't say that they will go self ID. I looked at their policies couldn't find anything.
They could change the policy quietly get a yay from Twitter. And just keep quiet about it in the hope that it won't come up in real life iyswim.
Fyi I had a look at swim England again. They published a self id policy ages ago like 2019 or something. Outcry (mixed changing in communal rooms aged 3- 70).
The page for the policy is still 'under review' now. They've backed away from the whole thing.