I can see that teaching that 'transboys have periods' might become necessary for the sake of some very confused children who believe that their inner gender ID does outweigh their biology.
'Yes, Ollie, your inner gender ID is a boy, but really, you do need to know that because you are female, you will menstruate when you hit puberty. Here's what you need to do about it.'
'Louisa, I know you ID as a girl, but you aren't going to menstruate because you are biologically male and don't have a uterus. Instead, here's what will happen...'
Could that be what they mean?
I keep forgetting, I think, that although we know what happens in real life to boys and girls, the children in the thick of this don't automatically know about puberty and sex. Those brought up with alternative facts may genuinely find it hard to work out which biological facts will apply to them.