I guess I read the letter as saying he went to them for help as a young child and they denied and gaslit him about it all along, which as you say would be abusive in itself. But on rereading I think the letter implies that he's only brought it up to them as an adult, perhaps for the first time. If they never knew in the first place, so they didn't do anything wrong at the time, then yeah, it's bad but not at all on the same level. It seem the letter could go either way.