This is correct:
the registry won't let nameserver in use be deleted
However, imho, this is incorrect:
as the nameservers weren't created here, we can't display them in your control panel
I had similar problem with "main" tucows once, but my ticket was taken care of. They simply added missed nameservers manually. This rep. Martin likely did not have such an experience before, but the current registrar (tucows) is able to manage existing nameservers under the domain in question, in all aspects, and obviously it is possible to display those to the customer.