We use them during hardware development to look at the waveforms in detail well beyond what is needed to read the bits.
The reason their interposer doesn't work with DDR5 is because they designed it with DDR4 as the target, not because DDR5 is impossible to snoop.
What do they actually look like and are there teardowns that show the analog magic?
It's a thing. It's expensive though. At some point you copy-paste scopes and trigger sync them.
Edit: https://www.teledynelecroy.com/oscilloscope/oscilloscopeseri...
I wonder if these are full sampling scopes. In the past we had Equivalent Time Sampling scope(wideband front end, fast sampling slow rate ADC, a variable delay trigger) and many buses have repeatable test patterns that let you trigger that way. They were always a fairly niche device.