You can break through a health gate for a limited period of time. At the start it’s barely possible, but as the slayer buff increases, the more you can break through them and even finish the primeval before they go immune again.
I don’t think I’ve seen anyone do it after one or two waves of envoys. I always assumed it was not possible on the first phase and near impossible on the second. I could be wrong though. Maybe there are some teams who are very good at coordinating their highest damage just as they are about to hit a gate.
The only time my teams melt bosses from high health is after invasions have healed it and we have a high slayer buff.
The health gating is to allow for a comeback. Killing enemy guardians while invading will regen boss health. Killing waves of primevals will give an increasing damage buff against the boss. This buff after a number of waves will let you just melt the boss and win.