Well, I did call the technical support to inquire about how the 16.5V "boost" desulfation actually worked. The person definitely knew more about battery tech then the dude(s) in all the videos. I, on the other hand, could obviously not keep up with him. When I tried to parrot then line about pulsing for desulfation, he got slightly defensive and claimed that rapid pulsing isn't the only way to desulfate, but that very low current at higher voltage also works. He also claimed that the charger was outputting variable voltages during the desulfation process based on sensing, algorithms, etc.
At the end of it all, we basically concluded / agreed that I recovered some capacity but probably not a lot; and a test with heavy loading would be required in order to know the state of the battery with any reasonable amount of confidence.
Anyways, if someone more knowledgeable than me (ahem dwh) wants to have a chat, their open until 5PM east coast time and the number is 1-800-456-6626. I think you press 3 for tech support.
EDIT to add: Drove the rig to work & back; parked, and a few hours later measured the voltage and got 11.5V. Ugh. Disconnected battery negative from ground post and observed sparks between the two. And the ground post showed +11.5V relative to my battery negative...and electrical continuity between ground post/engine block and the battery positive...WTF??? This persisted for a minute or two, then it stopped. Seems like some shorting. Sheesh...forget the charger/desulfation stuff...seems I got some real work to do.