I am suffering with the same problem.
I have the same type of saddle on all of my bikes and am fine for 6+ hours outdoors.
On the turbo trainer I have to stand for 1 minute every 10 minutes to keep numbness at bay!!
I think that it must be due to the bike not moving about much on the turbo trainer compared to out on the road.
I would like to do much longer turbo rides but for now I am keeping thm down to 75-80 minutes.
I think that this is a misconception. Very few, if any, cyclists will stay in one position while cycling outdoors for a prolonged period. They stop for junctions, get out the saddle occasionally and generally move around. While on trainers they are focussed on the session and it is completely different.
Certainly true for me at least. I regularly do sessions of 3 hours on the trainer but I move around a *lot*.
I regularly do 2hr sessions, I experimented with longer during lockdown but they weren't nice.