This is what I personally think HPZ was meant to be at some point. Get 7 emeralds, and the next star post would send you to Hidden Palace. There you turn Super touching the emerald or some kind of event there and the level ends after reaching the top of the infamous slope, which by the way and IIRC the speed set for super sonic allows him to go through it without problems. That would also explain with you can't go through this slope usually, and why the level didn't have a "normal" end. And it also makes sense considering this is somehow what happens after collecting the 7 emeralds past MHZ in S3&K. Well, I guess we'll never know; probably nothing of this was ever coded anyway.