Yes, it is. If a rider hasn't had the saddle time to gain some sort of fitness, they likely haven't had the saddle time to develop group riding skills. Does a rider know how to hold a wheel and hold his line? How to eschelon? How to pull through smoothly without causing a gap? How to fill a gap smoothly? How to bunnyhop an obstacle? How to look back without swerving? How to stand without surging back? How to eat, drink, take off or put on clothing?

All of these skills require a lot of saddle time, a lot of practice, and a lot of reinforcement. I've been doing the local group rides for years, but I still feel like a squirrel if I'm on a ride for the first time in a month or so.

So yes, it's a broad assumption, but due to the potential calamity of an unskilled rider in way over his head, I would make that assumption every time until shown otherwise.