Sure, you could end up being a doctor, or a ditchdigger, but there’s such a thing as a military doctor, and historically, various armies have always been the world’s number one employer of ditchdiggers. The catepillar thing is obviously a joke. Logistics/intelligence is also a military role, and we’re never told what sort of research Carlos was doing, are we? I’m still not convinced. Like I said, there’s plenty of non-combat jobs, but non-combat does not mean non-military.
If Heinlein ever did complain about people not getting the book, I’m pretty sure it wasn’t in Expanded Universe. So far as I know, I’ve never read any specific comments by RAH on the matter.