It absolutely is a glitch without any shadow of doubt, unless considering it a hidden Exotic perk of Wardcliff Coil. It's both an exploit and a glitch because it provides an advantage, though for a variety of reasons it doesn't have a lot of legs in competitive terms.
To clarify, a single Coil ammo round becomes full power ammo for any other gun. This is almost undoubtedly because of how the Coil technically stores a huge volley of projectiles per "rocket." This is unintended behavior and does not occur with any other gun.
personally I don't remotely care. the number of rockets one player has in reserve isn't usually the deciding factor for success vs. failure on that encounter and they had more than enough stacks to do it without. if it's "don't use any glitches at all" they should be disqualified, if this is kosher, specifically since literally anyone can do it and it doesn't break the encounter itself like the Calus-specific glitches/cheese, then they're good. again cannot bring myself to care at all