So, I guess the statement "replaces the crystal with a vacuum tube" is a little misleading. I wonder why not simply use a Tentlabs clock instead.
There is a crystal in there, it's the blue wrapped thing.
Looks like there's a coil of wire under the shrink wrap, acting as an
"oven", keeping it at a constant temprature for stabilty (35C. is
typical). The tube triode doesn't suffer from thermal changes in the
junction the way a SS device does, making for an extremely stable clock.
This actually very old technology - on the order of 50 years old, just
used in a different way. It was invented about the same time as
single-sideband (SSB) communications, where even a 10Hz drift in
operating frequency was noticeable on the receiving end.