Disclaimer: I put a big chunk of that paper together.
You ask how it can be guaranteed with "general code". The first thing to remember is that retpoline is not for "general code". Linux, for instance, does not support arbitrary call depth and barely uses recursion.
Also, take a close look at the "Exploit Composition" section of the paper. Those five conditions are much harder to satisfy at 'RET' than they are during the demonstrated Spectre variant 2 exploit points. For instance, a long speculation window (#5) for 'RET' is interesting to generate since it means a stall while waiting on something to come off the stack.
You ask how it can be guaranteed with "general code". The first thing to remember is that retpoline is not for "general code". Linux, for instance, does not support arbitrary call depth and barely uses recursion.
Also, take a close look at the "Exploit Composition" section of the paper. Those five conditions are much harder to satisfy at 'RET' than they are during the demonstrated Spectre variant 2 exploit points. For instance, a long speculation window (#5) for 'RET' is interesting to generate since it means a stall while waiting on something to come off the stack.