I think the joke is that Air Marshal Kenneth Porter was chief of the RAF’s maintenance command, and also not a pilot. [0]

Edit: fergal_reid’s reply below is correct. Porter did serve several years at the start of his career as a pilot, in the early 1930s, before spending the bulk of it as a signals officer. Holden also had his wings, but never served as a pilot. I read Porter’s comment as a joke between two RAF officers who were not “really” pilots, but now I’m not sure how to read it.

0. https://en.wikipedia.org/wiki/Kenneth_Porter_(RAF_officer)