“After reaching max-fire velocity this bullet could only gain speed with gravity.”
Completely false. A bullet slows down from the moment it leaves the force of the barrel.
No argument from me.
Not to put words in his mouth, but I believe he's saying after the projectile has passed it's apex, it might regain some velocity because of gravity. This of course would be assuming a fairly significant launch angle.
But, you're right - as soon as it leaves the barrel, it begins to slow down at a predictable rate.