This, by definition, isn’t a “premade”. Doesn’t qualify for it. It’s just a randomly matchmade group communicating via voice com.
A premade is a BG team made beforehand, with control over setup, gear, talents, skill, obedience (as in ability to follow orders), and that this is done before queuing for the BG, thus the term ‘premade’. Because it’s a team organized prior to the BG itself.
PS:
There’s already voice coms in-game, which fills the exact same role as your suggestion for a discord without trying to organize a premade.
Shame horde can’t actually organize premades for it, the matchmaking pool is both too large and the queue times are too long for it to work. You need at least one of these requirements to perform the “premade trickery” with the matchmaking mechanisms.