Am i missing something here?
Errr ....yes.
On a Cat IIIb (no DH) approach there is no requirement for you as the pilot to check/confirm/verify by eyeball ( or "observe" as you put it ) that you've got 75m RVR as you touchdown, or do anything about it if by chance, on landing, you just so happen to notice the vis less... wizofoz explains why below.