Here's what is says in the stipulations, Section 3:
"g. a properly-sized whitewater type I, III, or V life jacket (approved on the label for paddling, whitewater, kayaking) in good working condition for each member of the party."
Sounds like the issue was with the "approved on the label for paddling, whitewater, kayaking" part. Its surprising that a whitewater PFD didn't say so on the label.
I've had rangers warn me because the labels on some second-hand Type V PFDs were getting worn and wouldn't be legible with a bit more wear. This was even though the PFDs were obviously outfitter grade and still in great condition with no tears or worn out spots.
Here's the link to the stipulations:
While its easy to slam the ranger for being a stickler, if its Alvin, remember that he's seen a lot in that canyon & he's probably been on a body recovery or two there or elsewhere. Not only that but there may be pressures on him from above (management) to make sure everyone's following the rules to the letter.