Well if the defense really is unsustainable, that's probably valid. Until Duncan went down, I didn't think it was unsustainable. Now I'm doubtful that they can keep it up. SpursFan86 mentioned 3PA/FGA. That's the stat that shows their shot distribution - not their raw number of 3PA. And, yeah, that's pretty light.
That being said...
What team do you think has the most FGA's per 100 possessions? Yep, the Spurs.
The most FGM per 100 possessions? The Spurs.
The real problem for the Spurs is that the numbers 2,3, and 4 teams are the Warriors, Thunder, and Cavs. And THEY put up a lot more 3-pointers per 100 possessions than the Spurs. That represents probably +5-6 PPG in those teams' favors. The other teams in the league, the Spurs' shot distribution is fine for.
That means that the Spurs are either going to have to put up more 3's against those three teams, or they would have to be able to stifle them with superior defense. As of now, they can't do the latter. And, yeah, if it's putting up more 3's, they have to make some of them.
My point is, the Spurs are putting up enough 3-pointers to sustain their record. But against the 3 other top-tier teams in the league, their shot distribution puts them at a disadvantage.