I feel like it's an interesting intellectual exercise that you are positing, but I think it's important to read that
1. The rule doesn't specifically say they are part of the unit. The second sentence of the rule clarifies that they act as a separate unit for the purpose of basically all rules despite being in unit coherence but IMO it doesn't imply that the first rule's unit coherence is because they are truly one unit. It's definitely RAW in some ways, but I feel like RAI is that drones set up in coherence but are still always a separate unit except for Force Org Chart purposes (2 units for the price of 1 force org slot).
2. Unit coherency is normally 2" or all within a vehicle together, but part of the reason for the vehicle rule is because embarking and disembarking happens at the unit and not model level.
3. Further complicating things "When y ou set up a transport, units can start the battle embarked and within it instead of being set up separately" (source: enhanced digital rulebook last page of the core rules before examples start)
Therefore, I propose that RAW combined with at least part of RAI (the general intention of drones as separate units) would read as follows:
"The Drones are only treated as the same unit as the pathfinders for Unit Coherency on deployment and Force Org slots. Therefore 'any accompanying drones are set-up within unit coherency of [the pathfinder team]' can be read as simply requiring that the drones be within 2" horizontally and 6" vertically of any pathfinder models which are not on the field.... but if we aren't *BAD WORD DELETED* we can interpret that as not allowing deployment anywhere but rather just within 2" horizontally and 6" vertically of the transport for the first drone placed then 2" horizontally and 6" vertically from all previously placed drones and the transport for subsequent drones because we are interpreting "a unit must be set up and finish any sort of move as a group" to not include the drones in general because the drones are a separate unit and do not need to declare being embarked on the transport even though they must maintain coherency as the drones are a separate unit from start to finish except for force org and set-up coherency purposes, but they can be set-up within range of the model representing the pathfinders (the transport) so they can maintain coherency without being embarked before hand as if they truly were one unit".
To further this, I propose another question: Models in transports are not considered 'on the field'. Yet they are set up in their devilfish, so a unit deployed inside a devilfish with drones, do the drones count as part of that unit until they disembark as they're not on the field, or do the drones count as a different unit as they're already 'set up', or do the drones, having skipped being 'set up' and 'on the field' in the same action, never get considered 'set up on the field' and so never separate from their parent unit? Obviously, RAI is probably the second choice, but RAW I could see it being argued, not very well, that it is the third choice.
While I don't remember reading a FAQ about this specifically, a key rules consistency I noticed in the explanations of FAQ questions about vehicles was that they are still units. Therefore I would propose an alternative which is that they are set-up on the field, but only represented via the Transport's model. The transport is set-up and the units embarked within it count as set-up at the same time is the obvious intention of RAI as the exact wording of the rule includes the phrase "instead of being set up seperately" which I think implies that even with RAW the setting up of the transport also counts as setting up of the embarked units (with the only exception being that it still only counts as a single unit placement for determining who goes first). On the other hand, as weird as it sounds, I would say that if you embark the drones they must disembark with the pathfinders, however I think the intention of the rule "cannot both embark and disembark in the same turn" is for that specific ordering and so it would be totally kosher for the drones to disembark with the pathfinders and then immediately re-embark or for the pathfinders to re-embark after disembarking to leave the drones behind (which could be conveniently skipped as long as there's a reasonable expectation that the 3" disembark radius will fit both units in their entirety without being with 1" of enemy units). This is because the Pathfinders are simultaneously set-up but not set-up in such a way that allows coherency to be established or rather they have to be set-up with coherency and I'm not sure they count as coherent in the devilfish. Or something.
Of course, I don't, ya know, actually think that. However, I would say it would be a reasonable reading of the rules and an example of how treating Drones as a single unit with Pathfinders would be ridiculous as such a reading/ruling is easily avoided by simply assuming that the two units are never the same unit and that the second sentence in the description of "Drone Support" is simply an explanation of the fact that they are not the same unit except for, as mentioned before, force org and initial placement concerns at deployment.