I can understand his point of view. WWE has the performance center, a house show loop, and frequent dark matches. AEW has, reportedly, training seminars by the likes of Dustin Rhodes and such, and the Tazcalibur Dark Podcast before that got cancelled (which has been universally agreed to be a poor choice, hashtag bring back dark, don't @ me). But more importantly, the Tazcalibur Dark Show meant that the young rookie talent would have a semi-regular place to get some reps in. Sure you can work the indies while you're with AEW, but there's a difference between being told you've been booked to go ten minutes in a back and forth with Serpentico at this exact date and time on Dark, and being essentially told to 'hit the indies at your own time.' Which for someone with little to zero experience or indy reach, I can imagine being daunting and difficult.
Out of the early AEW rookie talent that popped up, Brock was... Not particularly a standout, to be polite, but that doesn't mean he couldn't grow into more of a wrestler. He needs that structure and direction to grow, is all, and AEW has a philosophy of 'you grow at your own devices' freedom sort of way. Different way of teaching or learning, some do better being led, others do better experiencing it firsthand. Nothing wrong with either path.