The “unrealistic” descending speeds was referring to Zwift. As I’ve already said, there have been plenty of discussions about this already.
For example, this (rather long) thread is quite interesting and covers some aspects of this such as supertucking effects, group churn, descending “skill” and rider weight (Pack Dynamics Test Events (December 2022) - #920 by Warren_Shiau).
I’ll leave it here, for fear of monopolizing this particular thread and taking it even further into the weeds 
.