I had a suspension seatpost too and hated it. Sitting on it feels awkward, a bit insecure.
On a MTB it won't work at all.
I suspect those were just invented so that salesmen can tell people "No it hasn't got a rear suspension, but it has a suspension seatpost that works just the same!" - which is bs honestly, because a suspension-seatpost won't help at all in difficult terrain and, as the others pointed out, you really shouldn't sit on your butt there anyway (but even if you would, it's not the point of a rear suspension to keep your butt comfortable ).