They aren't doing that. Al Jefferson is the worst defensive big in the league and would improve the Spurs offense marginally at best while making the defense significantly worse.
No his defense is not serviceable he is worse than Boozer and thats saying something. Dude is so overrated it isn't even funny. He gives up more points on the defensive end than he scores and he is such an overrated scorer. His scoring efficiency is decent but nothing special and the Spurs are a hyper efficient scoring team. His defense is abysmal and he is a waste of cap space.
The Jazz gave up something like 10-15 points per 100 possessions more when he is on the floor. Is Jefferson going to make the Spurs offense 10 points better per 100 possessions? Maybe 5 at the very best.
The Spurs aren't dumb. Just a bad rumor.
Btw valuewise Splitter>>>>>Jefferson anyway. Will probably have a much better contract, averages 15-16 ppg per 36 on very good efficiency. So-so rebounder but his defense is 10x better than Jefferson's.