Limitations to training on singlespeed?

Why aim to ride in a group with strong riders?

It makes you work harder.

It develops your technique, both because you are forced to refine your style to economise on effort, and because you have the opportunity to observe the skills that the others have already perfected.

There are too many variables for this to be anything other than conjecture. Group riding has it's benefits, but it's far from the be-all and end-all of training.

All but one of the things you mention above can be worked on alone.

Anyway, we all know "training for sportives" means a 3k carbon bike and getting an expensive program off an online "coach" who you never meet.