Having been to a furrycon a couple times in my life, I dont think he would've been kicked just for wearing a diaper under his clothes unless he was running around in a nasty used diaper being a biohazard. From what I've seen babyfurs can do whatever as long as they are 1. Not creating a health hazard for others 2. Are covered up in public areas i.e. not running around in just a diaper since thats more or less fetish gear in this context.
Furry cons also do usually have babyfur and other 18+ events where I assume the rules are a bit more lax about dress code, but you have to cover up once you leave those rooms (which have a staff member at the door to ID check and remind ppl to cover up before leaving).
Basically if this dude got kicked from a furry con I doubt it was even just for wearing a diaper under his clothes, he probably was running around in public in just a diaper or some shit and/or being a hostile, nasty, belligerent, and possibly violent asshole. If the Tranch was one of the guests at this con I think it's safe to say any claims of transphobia are completely BS.
Seriously, how bad do you have to be that even kevkev doesn't want to deal with you? New to this cow but holy fuck.