I didn't know that. After some reading, I see they found no red slag glass while digging the site, leading to that conclusion. That's what I like about marbles, always something new to learn. If those were amber or blue, they would probably easily be attributed to MFC with those 9's on them. They look like they're on top though, unlike most of the MFCs I've seen, so maybe not.
I've never found a peach or vaseline slag in 100% mint condition.