It’s not just a definition thing. I’m not sure if you read her essay but there’s a portion where she questions why a lot of transgender people are friends with other transgender people.
Her answer? It’s because they’re not really transgender, they’re simply being influenced by peer pressure.
She’s simply repeating the whole “you’re not gay, it’s just peer pressure / a phrase” except with trans people.
People are making this out to be a definition thing but it’s more than that. JKR is really fixated on the whole bathroom issue, and on the idea that a lot of trans men aren’t really trans, they’re just confused.
She also has this absolutist view that providing support for trans women will take away support for “real” women or devalue them, etc.
Contra point has a really entertaining and long video that examines JKR’s total views on this issue. It goes more than a simple disagreement on definition.