Wow... what kind of ADCs go into a thing like that? I'd love to see a teardown. Or atleast a price tag.
The only way to sample a 100GHz signal is to interleave a bunch of slower ADCs. How fast those ADCs are simply changes how many sample-and-hold buffers will be required.
The real question should be: what sort of sample-and-hold circuitry goes into a thing like that? That's the really tricky part.
As for the price tag, Lecroy's 60GHz scope starts around 500k$ so I would expect a 100GHz equivalent to be in the 750-1000k$ range.