Totally get what you are saying. I'm not a regular listener of TFATK recently but I've heard perhaps 20+ episodes over the years, occasionally I check in on it.
I feel pretty confident in reading that Schaub meant to infer it was a challenge to pass USADA tests. He has specifically called out virtually the entire sport (of course not him, but everyone else) for being on PEDS on the show... so this isn't anything new.
Most likely Carwin doesn't appreciate Schaub being what is essentially flippant about his qualities - eg. 'he hits hard, and can do good because the heavyweight field is SO awful, but he has terrible cardio, and Cain stilll whoops him.'
It's not exactly what you expect of "friend," certainly not a close friend.
Whats worse - he has to suffer this from a friend who is most famous for getting KO'd by mid tier fighters, not that he is aware of it.
Brandan (for sure t-shirt empire, for sure successful bro, my porche, Carwin has no cardio) is the type of dude that can be brash and uncouth, certainly out of touch.