I think it's still going to be an uphill climb for the Spurs, to be sure, but I'm less convinced than others (perhaps) that the 2012 WCF exposed some significant gap between the teams.
There's plenty of reason to think that if Harden doesn't make a three deep in the clock with under 30 seconds to go in Game 5, the Spurs re-seize momentum and -- at the very least -- force OKC to win a road Game 7 in SA. Ifs and buts, yeah -- but in the broader context of the series, Harden's three ended a 13-2 Spurs run that covered more than 5 minutes of the 4th quarter and which had cut the Thunder lead from 101-88 to 103-101. Great players on winning teams make that shot (and make it on the road to stave off a monumental collapse); Harden keeping his team from going down the drain is part and parcel of him being a great player (Harden also had 12 of the Thunder's 27 points in that 4th quarter). But if that one rims out, there's reason to think the Spurs win Game 5 and re-inject a little doubt back into the Thunder; as it was, having staved off that run to win a crucial road game had to just increase that team's confidence.
I'm not saying the Spurs were the better team or that they were screwed somehow; I'm just saying that the reality of the situation is that the Spurs were closer than many give them credit for in hindsight -- as this board well knows.
Notwithstanding that, one thing that I don't see mentioned much but which might be significant if the Spurs and Thunder meet again is the fact that several other teams have tried to mimic the Sefalosha-on-Parker concept in trying to slow Tony; aside from the fact that the Spurs can try to be better prepared to handle that sort of a switch if Brooks chooses to use it again, Parker has had to deal with it and seems (from my vantage point) to have figured out some ways to overcome the size of bigger defenders. Sefalosha is a different creature because not every long guy who gets assigned to Parker has Sefalosha's defensive gifts, but Tony's at least getting a look at that problem and seems to be finding ways to maintain his effectiveness in those situations.