I mean, he was rated as a top 3 dman of the draft before his draft year, but fell because he didn't put up gaudy numbers (because he didn't play PP1). I think scouts relied too much on counting stats.
Like, I see no reason why a guy like Quinn Hughes goes top 10 but Casey falls to the 2nd round other than stat watching.
Casey was under-drafted for sure, but he is not really comparable to Quinn Hughes. Hughes has two skills that are 95th-99th percentile in the NHL, his skating and his game sense. Not saying Casey isn't good at either, but just not at the level of Hughes.
I believe if Q Hughes were the same size as L Hughes he would have gone #2 in his draft, behind only Dahlin.
Getting back to Casey, I still think he's got a chance to be a Dan Boyle type player. The game is more open to smaller defenders now than it used to be. There is certainly a chance he fizzles out due to size, but he's got a lot of talent and could help out a teams PP, just have to find him the right type of D partner.