To "write a check (one's) ass can't cash" is poor-folks slang for making promises or boasting, then failing to deliver on same.
I think the confusion stems from the OP's use of the possessive. I'm guessing he meant "asses".
One could simply have said "Fucking HP, right?", and there would have been no confusion whatsoever, because fucking HP, right?
I had the exact epiphany, myself.
Heinlein had lots of ideas, many were great. Some were silly. Having consumed most of his books as a kid, I must say, they did me no harm.