To be precise, the exemption is 'in signed bays', not marked bays. Signed as in 'parking place', not 'marked' as in exempted footway parking (in London).
The on-street confusion is that the driver was in a marked bay, but the sign in evidence simply repeats the blanket restriction which is neither necessary nor what a driver would be looking for.
IMO, the TSM makes it clear that where markings exist then there is a presumption that the blanket restriction does not apply. Having been disapplied, other traffic signs, of the form prescribed for parking places should be placed.
IMO, it is NOT permissible to have what are effectively RPZ repeater signs within a bay because they are not traffic signs prescribed for parking places and therefore in the absence of any such signs the markings indicate a bay where the RPZ restriction does not apply and the absence of parking place signs means that parking is permitted 24/7 without payment or other conditions.