Here's a link to the warranty document:
http://powerequipment.honda.com/pdf/warranty/p-pwl50623-q.pdf
On page 2, it does state "consumable items" (blades, belt, tires etc.) are not covered as they are wear items. I don't see anything about a "1-year warranty on the belt" and if it just popped off after only one year of residential use, I don't see how that is even a wear item, provided the belt was in good condition and simply needed to be re-installed.
At this point, I'd suggest you contact Honda Customer Relations and get the straight scoop. Have your frame serial number handy and call 770-497-6400. They can help resolve this and are the authority for warranty claims.