Phil, I don't know except that Mex is now saying low and slow is ok as long as you get to 100% if you start at a high enough stage to be doing shallow cycles, but use 20% "rule" if you have a Bulk stage.
The AGM guides do say that using a faster recharge reduces time for the batts to sulfate. IMO that is sort of bogus, because there is little time for that to happen once you do start a recharge, even a slow one
Mex has said something about forcing the electrolyte into the mat better with the higher proper voltage and ? also the higher amps? IIRC
LY has explained how his AGM loses capacity with solar low and slow, but that is mixed up with also not getting to 100% each cycle before dark.
Anyway, you know your routine is working for your AGMs, so no worries what the guides say exactly.