As an aside to the conversation, our local USCG inspectors for a while required our dive boat to have RAM lights for for night diving. We tried to explain that we are always at a mooring when night diving, thus we would have our "anchor" light lit. We could not win the argument that being at anchor was higher on the chain than being RAM. So we spent the money to have a removable light assembly made that we never used. The following year a different inspector rotated in and we were never asked about the lights. Money wasted.
Not to split hairs infinitely, and agreeing that it seems redundant, but your previous inspector appears to have known the rules:
Rule 27, Vessels Not Under Command or Restricted in Their Ability to Maneuver, Section(b): "A vessel restricted in her ability to maneuver, except a vessel engaged in mine clearance operations, shall exhibit (subsection iv) when at anchor, in addition to the lights or shapes prescribed in subparagraphs (b)(i) and (ii), the light, lights or shapes prescribed in Rule 30." (Rule 30 being, of course, Anchored Vessels and Vessels Aground).
I think the rule reads this way so that a vessel such as your dive boat may declare to another approaching vessel something to the effect of, "Yeah, I'm anchored, but even if you've got a problem with that and think I shouldn't be anchored here and that I ought to weigh and move, I don't have to and you shouldn't ask, because I am RAM owing to the nature of my employment."
Maybe diving operations have to take place at night in a navigable channel where an approaching skipper thinks, "who does that $#%&@ think he is, anchoring in the channel?" With RAM lights, question answered, and no one has to pick up the radio.