I think we're just talking in circles and getting too into semantics. I think you are conflating "matchup well with GS" with "It's a mismatch". Other than that I don't put too much into those regular season meetings, I don't really disagree with anything you say. Obviously sometimes there is relevant info to be pulled from regular season, sometimes there isn't. No 1 size philosophy fits all.
When people say OKC is a bad matchup or matches up well, they just mean they're the best team in the league that has he size, athleticism and versatility to match up with the Warriors on the defensive end. They're the best offensive rebounding team in the league which makes going small lineups harder. They can switch the pick and rolls rather than have to trap, Westbrook at his highest level is as good as you could be guarding Curry and if you're not hitting 3's then those long rebounds off the rim lead to Westbrook in transition and nobody is better than him at that. Defense creating offense.
I don't think people think OKC is LeBron and GS is JJ Redick. Just that in their vastly improved state, they obviously present them some problems that other teams wouldn't be able to.