It's probably hard to admit someone did something better than you

He talks about how he'd had enough of metal, Testament were not going to be a headline band and he'd never played a headline gig yet Transiberian Orchestra (or whatever their name was) offered him that chance. It seems clear to me Alex didn't leave Testament because he was better, he left because he thought he deserved more. Fair enough to someone who thinks like that but saying anything derogatory about your replacement in the band you didn't want to be in is a shitty thing to do.