Am I wrong?
I would say, yes.
More brood is more winter bees and they need somewhere to go, somewhere to store further honey and pollen. If the poster is in the Uk, I would definitely not be constricting an already-full, nucleus sized colony to a nucleus box from now until next March/April! Even an extra couple of frames of stores is going to be advantageous - and we may possibly (but must say it's not looking too good at the present) get a good long autumn yet.
If there are frames available, they will build comb for winter feed to be taken down, if the weather is conducive.
They at least need the opportunity for brood expansion. I may be giving more comb and more brood from other colonies.
We are guessing as to location, hive type and colony number here as the poster has given little information. I would certainly not give a negative reply under those circumstances, without further questions. So a conditional, but not definitive, reply.
Further a properly set up hive will afford just as much, or more, winter protection than a nucleus hive - unless it just so happens to be a polynuc.
My advice is still move them up, give them a chance. More details may emerge later, and there is no reason, if one must, for not putting them back in a nuc for the winter. Like I say (and Hivemaker, I think) it is only mid August. The lack of forage in some areas does not necessarily mean it is time to cluster!
Regards, RAB