@prn Well, Dang! I have found some sources that refer to Convoke and Delve as cost reduction, but that seems to be clearly incorrect so I won't even link to those. I find other sources like http://mtg.gamepedia.com/Convoke or http://www.mtgthesource.com/forums/showthread.php?28957-trinisphere-and-omniscience that refer to Convoke of Delve as a "method of payment." Even magicjudges.org says "We can pay using Improvise, Convoke, or Delve after activating mana abilities as part of paying costs."
OTOH, the actual text in the comprehensive rules says:
702.50a Convoke is a static ability that functions while the spell with convoke is on the stack.
"Convoke" means "For each colored mana in this spell’s total cost, you may tap an untapped creature of that color you control rather than pay that mana. For each generic mana in this spell’s total cost, you may tap an untapped creature you control rather than pay that mana." The convoke ability isn’t an additional or alternative cost and applies only after the total cost of the spell with convoke is determined.
The corresponding definition of Delve at 702.65 also refers to exiling cards from the graveyard rather than paying mana.
So I still don't see that I have an actual official ruling to the effect that Convoke (or Delve or Improvise) is a way of paying the costs so much as something you do "rather than" paying costs.
Even though the Comp rules say that Convoke (and same for Delve or Improvise) "isn’t an additional or alternative cost" the wording of "rather than" still makes it some kind of "alternative" according to the normal meaning of the word "alternative". That's just what "alternative" means -- something that is "rather than" something else.
I'm still looking, but so far, none of the places I'm finding explanations of how these abilities work with Trinisphere, for example, are "official", or at least not actually WOTC sites. When it comes down to it, I'm uncomfortable relying on anything other than WOTC for an "official" ruling. (Regardless of whether "alternative" as used in the official rules does or does not correspond to "normal" English.)