I think it's because the frame rate is usually used as shorthand for the amount it slowed down, since a fixed playback rate is assumed, but yes, it drives me crazy and when I heard "shot in 6000fps" I immediately thought to myself, "well tell us what fps it's being played back in so I can tell how much slower it is!"

When the guy filming it switches his camera from 30fps to 120fps, 240fps, or 6000fps, he knows exactly what the result will be so he starts thinking of "6000fps" as a specification for how slow it is, which it isn't.