We now say not to use the archaic and sexist term "mistress" for a woman in a long-term sexual relationship with, and financially supported by, a man who is married to someone else. Instead, use an alternative like companion or lover on first reference. Provide details later.
'Secondary partner' is the preferred term in polyamory. Why are you promoting outdated monogamist euphemisms like 'lover' and 'companion'? Do better, @APStylebook.
PS if you can't tell whether this tweet is earnest or satire, don't tell people how to write about sex.
