[Agent Hedgehog]

I corrected a student recently when she said "we're no politicians" and now I'm asking myself what makes it wrong. (It's a direct translation from German)
My questions:
a. could one say "we're no politicians, but ..." similar to e.g. "I'm no expert, but ..." and
b. if so, is that because it's a fixed phrase with a variable slot for the noun in the first clause?
c. is the "no" permissible because it's used for emphasis and therefore disrupts the normal syntax?
Any informed opinions out there on the subject?
Thanks in advance!