It really depends on the nature of the incentive when it was signed. If Diaw were under 254 pounds in 2014, then those incentives started off as likely to be earned, so they would have counted against the cap in 2015. Then provided he didn't meet the goal that season, they would have fallen off the cap for 2016 (meaning this most recent season, not this upcoming one). In addition to that, the Spurs would have gotten back the incentive for 2015.
Let's look at it this way. Say Diaw's deal was a flat $30M/4 deal that included an annual $500k weight clause. That clause started out as LtBE, but Diaw never met it. The cap hits would look like this:
2014-2015: $7.5 Million
2015-2016: $6.5 Million
2016-2017: $7 Million
2017-2018: $7 Million
Total payout is $28M/4 and flat from Diaw's perspective but divotted from that of the Spurs.
Anyway, according to Spotac, Diaw's incentives were always considered UtBE, so the team got back no cap space from him failing to meet those goals.
I will say, though, that they also list Diaw's salary for this upcoming season at $6.5 Million. So I don't know what's going on with the real numbers.