As y'all might know, no OEM manufacturer is in the oil business, therefore they have to have a vendor to produce their respective brands of motor oil and other lube and chemical items for them (in the appropriately-packaged and branded containers).
In this case, ExxonMobil could well be Honda's "vendor-of-choice". This could even mean that they've signed a contract of that nature . . . or that EM will build the oil as the OEM desires at a "better" price . . . and that they'll market it via their marketing network . . . several possibilities and combinations thereof. ONLY EM know for sure!
Key point . . . EM might in fact build and distribute the OEM-spec manufacturer's motor oil to the respective dealer network, BUT the oil is built (as contracted) to the manufacturer's particular specifications if it goes into the manufacturer's packaging. The SPECS of the oil are more important than who might build it, so that a different vendor could be contracted next year or in a few years (depending upon the contract length). There might be several areas where the two oils would be the same or similar, but I suspect there would also be several areas where their specs are different--even significantly so.
The OEM's specs are probably a little tighter than those of the vendor's normal brands of motor oil, but that additional tightness might not be relevant except in certain situations and environments. For example, if the OEM-spec oil is more resistant to sludge in sludge-prone engine designs, it would be to the advantage of the dealer network to use it and have more satisfied customers as a result (potentially) than if they purchased an "aftermarket" motor oil that cost them less money to get. Just an example.
In the generally smaller batches that the OEM's oil might be built in, the vendor could be more responsive to sudden changes to the original formulations IF some issues developed with it. The larger volume of the vendor's line of products would mean they could not be as responsive to the OEM's needs in this situation.
One of the first shipments of GMGoodwrench motor oil came on a pallet imprinted with "Valvoline" on it, but the oil looked physically different than Valvoline motor oil you would buy in a quart bottle.
Just as with tires, Goodyear might own and build Kelly-Springfield tires, but that does not mean that the similar K-S tires are the same internally as Goodyear-branded tires. In many respects, only their common ownership is the bulk of what the two brands have in common.
Many vehicle manufacturers have tended to have a particular brand of lubricants that they have worked closely with in their engineering and design over the years. In some cases, the OEM-packaged items might have been the same as the vendor's products, just in different packaging . . . but that was BEFORE we knew about oil analysis to see what was really in there or to reverse engineer the base stock formula.
Just some thoughts,
CBODY67