I also don’t think it’s entirely fair to say that SDKs were unknown to Sega at the time. They were being occasionally used on earlier consoles too (even if it wasn’t the norm). The real reason Sony had an SDK on launch and Sega didn’t was simply because Sega had a massive development resource whereas Sony had to rely entirely solely on 3rd party developers. So releasing an SDK was always on the roadmap for Sega but a lesser priority than it wouldn’t have been for Sony.
An SDK would certainly have made it easier to program for the Saturn but ultimately it would still have been significantly harder than the PlayStation.
Your overall point that there were lots of contributing factors is correct though. I wasn’t intending to suggest that the Saturns failure was down to one specific thing.
Edit: just to add, I agree there are some awesome 3D games on the Saturn. I own one and have a few. But the existence of good 3D games doesn’t negate the point that it was architecturally a challenging platform to do 3D on. I’d describe it really more of a 2D console that can do 3D via an official hack.
On top of this, Sega had some major unforced errors between harming their own credibility with the 32X and Sega CD and their surprise early launch that led to them launching with almost no games and angering retailers so badly that they refused to stock the system.
To me these are bigger factors than the architecture; the Saturn still does have impressive 3D games.