Yes it is the strong ridges on the sides of the rim of pouch. That is the only hint that make it Grande. If your plant is growing more vigorously than the usual caudatum, then that could be a longifolium influence.
I'd give the benefit of the doubt leave it as a Grande
I did post a few 'caudatum' photos with this sort of flare, which made me think. I don't know how valid those examples are. Many caudatum have the rims uniform to the rest of the pouch.
I think plant size was a bonus, and could have been one consideration to why it got a FCC. Your plant is special.
BTW my Phrag Giganteum (Grande x caudatum) is in bud, and will bloom in the weeks to come, I think the blooms will be similar. My plant has grassier foliage, but it wasn't my choice....internet order.