As a person who has worked at Dell as a firmware engineer, with the very same 1-wire connection over that third data pin.
Yes, it's true that at least HP and Dell will only allow use of an "authentic" adapter. The internal reasoning for this is that third party adapters may SAY they are compatible with system voltage/current, but in reality it might be insufficient or damage the board. (I personally think this is not a good enough reason, and that a BIOS setting with a safety disclaimer would be enough, but I digress)
The second and much more legitimate reason is that yes the adapter reports to the system what voltage/current it supports, so the system can set it's charging and power targets accordingly (e.g. if wattage is too low, the system will know to pull from the battery a well)
This data connection is proprietary for these three pin connectors (there's just not a standard, and no one wanted to negotiate one), so they just don't work between different companies. And this "accidentally" works as a vendor lock (I say accidentally because internally we always talk about it as if we are providing a safe service for customers, but I know the reality of capital so...)
What I said is true of Dell, and to my understanding it's also true of HP (as they both constantly try to copy each other, good and bad). So yes, both things are true at once.. it is in fact a design that is useful for these pre-usbpd systems, and very "conveniently" it has allowed vendors to lock in their adapters
That said, my understanding is that it's pretty easy to get a third party adapter that intentionally mimics that data pin, and they're much cheaper than the oem versions but of course buyer beware (personally, I've been horrified, completely horrified, at what I've seen on the insides of various cheap power adapters đ
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