It's the same system as in the other PvE modes I imagine. The lower the BR of your vehicle compared to the others on your team, the more points you get for things you do. You can be in a low BR vehicle and do barely anything, and still end up higher on the scoreboard than a high BR vehicle that does most of the work.

Same shit in air assault. Your slow F-80 with 4 50 cals gets put on the same team with mach 15 jets that can shred an entire squadron of bombers in a single pass. You end up accomplishing fuck all because your plane is literally not fast enough to get anywhere near the fight before most enemies are already out. You kill one plane here, get a few assists there, spend the vast majority of the game idly flying back and forth across the map and watching your teammates murder the bots from 8km away, but still somehow end up above the dude who has like a million kills.