No joke!
Well I don’t really know what “real” de-essers do, but mine could be described as a multi-band brickwall limiter, with many narrow bands in the default settings, which account for the slowness but perhaps also for precision of results if you can wait for it.
I hear it said that “sibilance” is eq’d down at 7 or 8 kHz, but I find that s sounds in my speech recordings sometimes have unpleasant piercing whistles in them that are much lower, between 3 and 4 kHz. You can easily see such things in spectrogram. I wrote something that does the program equivalent of scanning the spectrogram for such bright bits, and then fixing each one precisely over the right interval with the right gain using the Nyquist eq-band function.