[Sombre]

I think it's pretty clear what is meant. "Units which retreat are eliminated". Yes, they are under the normal non-siege-specific condition that there are no friendly provinces to retreat to. Something which is covered by the manual elsewhere. I guess it's /possible/ we are supposed to infer that the author assumes there are no bordering friendly provinces in a siege situation (which would just make it poorly written). But let's be real, the manual just has it wrong. You'll only find the statement ambiguous if you really go looking for ambiguity.