So Malcolm Thomas has never really contribute at a high level (NBA or best Euro leagues), is the 7th Spurs PF/C, hasn't played a single minute with Spurs and they guaranteed his contract. Spurs must surely be impressed by what he did in practice/D-League.
Spurs roster is quite unbalanced with 7 guards (Parker, Mills, Joseph, Ginobili, Green, De Colo and Belinelli), 7 bigs (Duncan, Splitter, Diaw, Ayres, Bonner, Baynes and Thomas) and only 1 SF with Leonard. While 7 guards could be understandable with Green, Ginobili or Belinelli playing also some SF, 7 bigmen is way too much especially since it's more 7 and a half with Kawhi playing smallball PF. Just a strange roster.
It's also noteworthy that even if Spurs have 15 players under contract, they still have the flexibility to go after a waived/D-League player they really like. Spurs are so far below the tax that they could waive an end of the bench expiring contract (De Colo, Bonner, Baynes or Thomas) and sign a new player without financial issues.