Yes, correct for "severe" turbulence. It protects you against stalls due to gusts (explanations can be found in the gust envelope diagram). On a B737 it will state you can only reduce to 250 in a descent if you are below max landing weight below 15000ft (250kts is not allowed during a climb). But I hope the person also applies the correct procedure for turbulence in cruise (on 737 classic that would mean flying in CWS and without AT, as ALT HOLD is not allowed).
The times I've encountered "severe" turbulence are very few... I think many use the speeds as a general limit for comfort reasons, but it is true you shouldn't reduce more as turbulence intensity increases.