Most B roads certainly are signposted on the ground.

Things like public rights of way references are expressly tagged differently, typically with the prow_ref tag rather than ref.

The problem here is that these routes have been tagged in a consumer-facing way (standard route relations) so that they are completely indistinguishable from signposted routes like the National Cycle Network or National Byway. A routing app will tell users to follow "HDC 7", and make its turn decisions based on that, when in fact there's no HDC 7 signs to follow.

If they were tagged differently then, ok, it still would be against standard practice for mapping cycle routes, but it wouldn't be causing active harm in the way that these are.