Re:
Sound like Leeds Mountain bikers group.
Also say knee pads should be worn, and for routes like Dalby Red, which is fine on the most knackered of retro none suspension bikes.
Also doable on a full sus activator as well.
It is probably just some house rules that group use to say, we're riding off road.
Some of the places may be a bit rough.
Hardtail was never a thing when I road properly, it was full or front sus as the option to a regular bike. So a hardtail to me is rigid, front sus or flex stem bike ;-)
Now it sounds like rigid just isn't a thought they can comprehend, but don't turn up on your road bike might be what they mean.
Ask them.
Why not get some rides with people on here, in these places.