OnePlus offloads heat to the charger

Some of it. They omit some circuitry that would have generated additional heat in the phone, and have it in the charger instead, but that doesn't magically mean the battery itself wont generate the inevitable heat caused by being charged faster. The battery itself only accepts one voltage, so the only way to charge it faster is amps.

And my feeling is that they aren't using the gains from this to make the batteries last, as SUPERVOOC is faster than pretty much every other standard. That makes me think they turned in any and all gains in battery health, for speed.

Most chargers send the additional energy via the cable in the form of extra voltage, because that doesn't require a special cable. Turning that voltage into amps in the phone produces a little bit of extra heat, but that doesn't mean that by eliminating that step, you get none from the battery itself as it charges. You can technically charge with a higher voltage, if you set up a phone such that it has more than one lithium cell. Some phones do this, but this doesn't require the OnePlus approach of using a special charger that provides a higher current, since any fast charger that can do the usual higher voltage method of providing extra power will work.

Like you say. I'm curious how they test this. Even if one battery gets more cycles, it'll degrade with time, as well. iPhones fast charge, too, but not with the chargers that used to come with the phones. You have to get one specifically for fast charging to get faster-than-normal charging.

Also, a tip. You may want to use something like AccuBattery to actually measure the state of the battery. Batteries, being chemical devices, have different capacities straight off the production line simply by virtue of not being chemically identically down to every molecule. (My Xperia 1 V unfortunately came with 93% design capacity, still within manufacturing tolerance, but the lowest I've seen on a new battery, it can be a bit of a lottery)

The built-in battery health monitor will just say "all good" until it isn't. AccuBattery has allowed me to monitor every percentage of degradation over the lives of my last few phones.