Yep. Can't compare the two because they are distinctly separate, non-overlapping descriptions.
One is a profession, the other a status or condition. A slave is presumably owned (though not all owned subs are slaves,) and if considered a chattel slave, presumably said slave can be sold. But that doesn't qualify as a purveyor of sex for money either.
A slave can be pimped out sexually to bring in money for the household or for the master's benefit, in which case, in addition to being a slave, s/he can be also considered a prostitute. But a slave can also be pimped out for money to humiliate and degrade him or her, presumably to feed a fetish of either slave or master, and though it would smack of prostitution, it still doesn't meet the criteria. It's still just a sexual kink.
There is no requirement that a prostitute be owned in any manner. S/he can be a free agent or can have a handler/pimp/madam/service.
All that aside... I suspect muskan isn't referring to the profession at all when he uses the term "whore".