These are good takes, and I mostly agree, especially re who they hoped he would be vs who he is. But I also see a lot of QBs making bad decisions and avoiding the middle of the field vs the Bills pass D. He's pretty much always sitting in the middle there. I have heard at least one opposing coach say that he's a real problem in pass D, and you of all people know that pass D is more important than run D (not that the latter is unimportant; it is). As for the completion rate, yeah, but a lot of dumpoffs to RBs go his way. As I mentioned earlier, Mixon caught 87.5 percent of the passes thrown his way, and while he's at the high end for RBs, RBs in general tend to catch balls at a high rate. It skews passer rating vs LBs, who let those passes happen and then rally to tackle. But he needs to be making 2-4 picks per season to be sure and breaking up more than the 4 passes he broke up this year (as opposed to 12 in 2018 and 9 in 2019). He's not doing that.
You'll probably laugh at this, but one skill he does have is running and chasing guys who've made short catches. He's very good at rallying to the tackle in open space (no need to shed blockers out there) because he's a punishing tackler and very fast for his position. He also only has to react and not read. Hence there will be catches against the Bills pass D, but he's good at preventing them from becoming bigger plays. That doesn't make him elite, of course, but it is an attribute that's probably more important on the point-prevention front than many give the team credit for. He had his best missed tackle rate ever this season - 8.5 percent, which is pretty solid for the LB position.
Re: math, you're missing my point. The passes that aren't thrown his way because he's doing something good out there will never reflect on the rating against him. My point is that teams this past season struggled massively vs the Bills pass D, and he's a core part of it in the mid-range areas. QBs get confused and make bad decisions, and it happened a lot this season. (I'm not saying he's all-world at pass defending, so please don't make it sound as if I am!)
Bottom line: he may get better, although I think he'll never be great. He also knows how to function in the system pretty well, and things can ALWAYS get worse -- the grass isn't always greener.
Interestingly enough, the LB I really wanted in that draft (after the QB, of course) was Roquan Smith. Man, is he good: https://www.pro-football-reference.com/players/S/SmitRo07.htm. This past season, opposing QBs throwing has way had an extremely low 76.8 percent rating vs him (LBs tend to have surrender very high passer ratings as a general rule) and he only missed 3.6 percent of his tackles. He is a difference maker. Doesn't force a ton of turnovers, though.