> So how am I supposed to interpret that claim?
As someone using a colloquial term for anything sexual without consent, obviously. Sex without consent is generally called rape; most people don't have any clue about the exact legal definitions. You don't even know what legal jurisdiction OP is in; how could you interpret the word as anything but the colloquial definition?
> In the US I don't believe there is any such crime as rape by threatening to fire someone, or if there is it is not a serious crime.
First, sex crimes are defined at the state level in the US, so it doesn't even make sense to discuss what "the" law is in the US. Pick your state, and you get your answer.
Second, if we pick, eg, Michigan, you're describing...hmm. "Third Degree Criminal Sexual Conduct", which is punishable by up to 15 years in prison. Not sure about you, but I'd call that a serious crime. Other states have different laws, but generally speaking getting someone to perform an act via the threat of firing is coercion, and penetration achieved via coercion is one of the more serious forms of sexual assault.