The pointer speed slider only affects the acceleration when the profile is set to adaptive, while it only affects speed when setting the profile to flat. This should be reflected in its name, also it might be good to explain flat and adaptive a bit more.
They both have basically the same effect though: they affect how fast the pointer moves. It's true that the manner is different for the different profiles, but I don't think revealing that level of specificity is useful in practice.
Mh, I am unsure. Without having a discussion in an issue at libinput, I would be completely unaware of what the flat profile does and that the slider basically adjusts the strength of the acceleration profile (which can mean speed _or_ acceleration, based on the selected profile). I also don't like libinputs current approach here, since it does not allow to adjust speed and acceleration independently. So this might not be a real issue of KDE, but it kind of "hides" the way libinput currently works from the user.