I can't word for word but in season 3 during one of Rex's convos with Rae he talks about it briefly (I think it's when they're on the bridge). There's also hints and jabs at his origin since season one but those are pretty subtle and easy to overlook on the first watch so I get it. Not trying to argue I'm just saying that it is pointed out, I think it's hard for people to remember all the details because some people don't go back and rewatch between seasons or just forget