This says that people understand indirect refusals the same in sexual and non-sexual contexts. It doesn't say that everyone understands them.

A person who never thinks "Shit, are they bored, or are they just making sure I'm not bored?" will never think "Shit, are they turning down sex, or are they just making sure I really want it?". A person who has trouble with the former may well run into the latter. (Still not an excuse though.)