Mistress has multiple meanings. Including 'being the other woman'.
But it can also mean:
A boss bitch in a position of power. More formally used as a title to address said woman.
"The Mistress of the household." "My Mistress, how might I serve you?"
Or:
A woman having an extramarital sexual relationship, especially with a married man.
Nowhere does it say *limited* to infidelity, though. So they are using the term correctly, if not the meaning you are used to. Extramarital just means 'outside of marriage', as 'affair' also has multiple meanings, including, very basically, a sexual relationship.
Cheating is not required for any of them.
Sometimes I wonder if these people even know the meaning of mistress? Like always in these damn harlequin, they just think that if you sleep with a rich man then you're automatically a mistress. Like come on! Ever heard of dating?!! Is he married? Does he have a gf?! Is she the other woman?!!! Then she's not a f*cking mistress!!!!