Here is a site of with what looks like direct feed screens from the game that are different from what is on that other site. Maybe it is by the same person, I do not know. But from what I can decipher of a google translation it seems he owns one of the prototype CD's or knows someone who does.
http://murakun5555.at.webry.info/201010/article_73.html
He's got it, and it appears to be a complete version of the game. It also seems to have been shipped to a magazine editor for reviewing since it has "please return" written on the disc. The guy doesn't even know for sure. He sounds like a collector who had no professional involvement with the game.
Here are some random old 2chan discussions where people seem to be claiming to own the game.
EDIT: Some talk like they've played the game. Nobody gives details and there are no follow-up questions. I dunno, this reads like a bad gamefaqs forum. I guess that's 2chan for you.
Anyway, I think there was a Sonic 1 prototype that surfaced in Australia years ago. The guy who had it said that he didn't want to incur the wrath of Sega and refused to sell, but I believe he also suggested that he'd like a five-figure sum when people were offering four.
The good news is that if it's a disc, there's a good chance the guy has it backed up. It's not like old flash memory that's vulnerable to bit-rot. He may have a change of heart and decide to release it yet. It also may take a few more decades.