While this is technically possible and valid (excluding something at compilation time that otherwise does exist in the development station), it would be so completely out of character compared to what they did put on all these builds that it's hard to reconcile. These builds are teeming with unfinished bosses, broken levels etc. It's very hard to imagine them going "let's leave everything we are working on as it is, no matter how broken, but also let's exclude Hidden Palace just to fuck with people 20 years later looking at this". I don't know, it's not completely outside the realm of possibility that they would have held back that which they didn't intend to work on further, but we don't have any evidence that they ever held back, and a lot of evidence that they often didn't. I admit the fact that Hidden Palace was removed before its final music was assigned to it is really fucking weird. But there has to be an easier explanation. And the thing is, even if the level was still present in development stations, they must have known at that point that the level was discontinued if they hypothetically went through the trouble of excluding it from the build. Which doesn't make it less weird that they would have reassigned its music after they knew it was discontinued. So, yeah, odd either way.