In the Spurs' minds, they expected to be hoping for a chance to draft a Primo who stayed in college another year. They'd have to decide between guys like him, Mathurin, Murray etc. Unless their evaluation has changed quite a bit since then, they shouldn't be looking to draft another SG. If they had the 8th- and 9th-overall picks this year and drafted Primo and Mathurin, they'd be insane. So drafting Mathurin would suggest either they think he's finally the star wing they've been aiming for, or that they're just going to keep taking those shots no matter how crowded the room gets.
What does that have to do with Williams? Because to me, if they are willing to just fill up on another SG, then they probably intend to just do that with center, and draft a guy who will be fighting with Jock Landale for gameday activations. I wouldn't be happy with them spending two picks on guys like that while they continue to have no forwards with size on their roster. Taking Williams at 9 would suggest they think he'll be their starting center within a year or so. It might let them trade Poeltl in an impact deal.