I'm not surprised, obviously. Samanic went on the chopping block after KBD was given a guaranteed deal. Giving KBD guaranteed money didn't make sense unless the Spurs were willing to cut Samanic or there was an impending trade. There was no trade so, yeah, Samanic had to prove himself worthy in order to make the team.
Samanic was a high ceiling, low floor prospect so this outcome was always a strong possibility. He needed to improve his work ethic, hustle and basketball IQ to become an NBA player ... and there was no proof that he improved any of those facets.
That said, I don't especially love the move. Even though he's still super raw, Samanic has a higher ceiling than someone like KBD. KBD at his best is like a solid 10th man. Samanic probably doesn't have a future in the NBA but his physical tools and youth keep the door open to him eventually figuring it out.
I would have preferred that the Spurs decline his fourth year option but keep him around for one more season to give him a real chance. , even if he continued to be clueless, he would have been an excellent end of the season tank commander
Cutting a halfway intriguing prospect that you've invested two years on before the first season he should have a clear opportunity to play? Eh, not enthused by the decision, IMO.