WHo was OKC's AB on the perimeter? exactly.
Thabo Sefolosha, who is a top-5 perimeter defender and who has more length than Bradley.
I agree with your greater point that just because OKC couldn't get it done last year doesn't mean the Heat are invulnerable.
What Thabo hasn't been able to do for the OKC is be that stopper and not compromise their otherwise stellar offense.
Though that's probably due to Thabo + Perkins in combination.
The offense has been clicking pretty well with Thabo in there this year. The Thunder have a 113.2 offensive rating while he's in there, only a small dip from when he's on the bench. The defense is markedly better, though (101.3 pts/allowed per 100 possessions with him on the court, versus 108.7 pts/allowed per 100 with him out of the game.)
I definitely prefer Bradley as an offensive player over Thabo, though. Thabo has a reliable 3 point shot, but that's about it.