IMHO, you are applying way too much retroactive continuity to identify the function of the component of a ship that's 100 years older.
This could be discussed in purely TOS terms, too - but the thing is, Star Trek nowadays features quite a bit of "bracketing", in which the technologies introduced in TOS and expanded upon in TNG are revealed to have existed in ENT already.
I admit that ENT never expands on what the domes in front of the NX-01 nacelles might be. But for them not
to be "Bussard ramscoops" (perhaps by some 22nd century name, but still) would be an exotic and unlikely twist, when every other piece of tech visually bracketed in fact gets dialogue verification for also being conceptually bracketed.
Nothing wrong with TOS representing a brief foray into different technologies, after which things return to the apparent ENT/TNG standard. The warp powerplant is a good candidate for a technology that had not yet stabilized at all, as it's visually so different in the 22nd, 23rd and 24th centuries. But the visual consistency of the nacelle domes does make me think this would be a good time to accept the bracketing argument as trumping the "it is old tech" argument. If there is a brief interlude with different technologies, it comes in TMP rather than in TOS.
but there's a rare shot of what I believe to be the "Phase II" Enterprise prior to TMP and this is a shot I had not seen before
This one has been previously printed in the Reeves-Stevens ST Phase II: The Lost Series
book. A beautiful design is shaping out here, but clearly a lot of work remains to be done... Todd Guenther apparently adopted this design as his Constitution II
heavy cruiser variant for the purposes of the Ships of the Star Fleet
booklet, acknowledging the work-in-progress nature of the model by having Starfleet "complete" it in a number of subtly different ways.
The way he chose to have Starfleet complete the nacelles was to leave the forward domes featureless - actually making them look more like TNG ramscoop domes than in any of the other preproduction variants of the TMP ship!
A clear hint, IMHO, we are looking at a design evolution towards the Bussard ramscoops of the TNG era...
Or we could say that the TMP nacelle closely resembles the VOY nacelle in that the scooping part is split and situated some distance from the nacelle bow (in those fingernail-shaped sections that are followed by the diagonal structure and then by the "warp field window"), whereas this Phase II nacelle resembles the TNG one, and the arrangements compete evenly throughout the history of Starfleet.