I had one of my school bullies try to add me on facebook years ago (nothing to do with being trans, this was waaay before I figured that out, but it was over a decade after I'd left that school) and it threw me off for days - I couldn't figure out why she'd even WANT to. I can only imagine it's worse when the scars are specific to coming out.
Best advice, ignore the request and don't feel you owe ANYONE an explanation if you don't want to give it.
Childhood bullies do so much more damage to their victims than anyone ever talks about, the scars run so deep they can be hard to even recognise for what they are - you cope with what few tools your brain has available that young. (Things I'm learning now about how bullying shaped everything about how I deal - or don't - with emotions).