They're not vacantists, they're plenists. They say the seat is occupied (not usurped) by legit Pope Bene.
Sound language matters.
Actually having been around here a bit longer matters. There was an actual thread devoted to this topic, and the name Bennyvacantist stuck as shorthand for my proposal: BennyplenesedeBergogliovacantist. Too long to write each time so it got abbreviated as Benny...vacantist. So the position is two-part, that the see is filled with Benny but empty of Bergoglio. It doesn't say enough about the position just to say that the See is filled with Benny. So it's shorthand for the proposition that the See is vacant of Bergoglio on account of Benny filling it.
Now, some Bennyvacantists interestingly emphasize that Bergoglio isn't pope on account of heresy (in addition to the fact that Benny holds the See). So my guess is that they might be inclined to sedevacantism if they didn't believe that Benny still occupied the See.