I really wish my response could be: " yeah! The Spurs might not be elite defensively for 48 minutes anymore but they are elite when it counts!

"
Unfortunately, those late game defense numbers are mostly a result of luck. Other teams missing clutch free throws against the Spurs doesn't have anything to do with defense.
And opponents three-point percentage has been proven to be mostly a product of luck, as well. I didn't want to believe this at first but I've read multiple research studies that basically prove that while better defensive teams hold opponents to lower three-point percentage, the margin of error when attempting to predict the actual percentage is so large that luck is undeniably the largest factor.
If you look back at San Antonio's rank league-wide on opponents three-point percentage defense, you can see there's little rhyme or reason involved:
1998: 8th
1999: 12th
2000: 24th
2001: 2nd
2002: 4th
2003: 7th
2004: 4th
2005: 25th
2006: 3rd
2007: 2nd
2008: 3rd
2009: 24th
2010: 6th
2011: 22nd
2012: 24th
The Spurs have been near the top over the years due to their overall great defense but there are unexplainable fluctuation that have nothing to do with the quality of defense being played.
So while it's great to see that the Spurs have been a clutch defensive team this year, it pains me to admit that I think it's mostly due to good fortune.
That said, if the Spurs defense can remain clutch even after the opponents three-point percentage and free throw percentage start to even out, then I could believe San Antonio having a clutch defense. Until then, though, it remains a hope and not reality.