Wow they were flying at 27,000 feet for a lot of that. I was wondering how they'd get over the Rockies but had no idea they'd go all the way up there. Obviously they need oxygen and the plane has to be designed to fly in that thin of air, but just how hard is that?

Not hard and the glider needn't be special; most already have a stunning glide ratio. I've been up in lesser wave in a clunky old trainer. You do need to coordinate with ATC to keep separation from jets. And the oxygen rig does has to be more serious than a nasal cannula above 18000 ft.

Of course then there's these guys going to 90000 ft ... https://perlanproject.org/

Yea, I was wondering about the Rockies, too, but it looks like they just got themselves high at the start and then kept high all the way.