This was momentarily glitched, or it was at least visually. It retroactively awarded it to people that met the conditions before it started tracking properly. This should not be an issue to anyone going forward.
I used an old solution from two years ago by MMMDI
This can be done with a fresh file of Lifeline mode.
Clear the initial zombies, get to the area where the objective tells you to. Don't actually go to the base though, drive past it (or go to the base and defend it so you can get the weapons, you may want them) and continue on the overpass route until you come upon an exit with a ramp down the right and drive down the road to the right. You'll see a fence area you should be able to fit your car through. Zombies will continuously spawn, so just drive around. When your car starts to flame, you can exit the area and go find another vehicle, a bunch should be scattered on the road right outside the fence area. Same thing if zombies stop spawning, drive out of the area and then come back.
Watch out for two of the special zombies called Freaks. The really big fat one that you can't run over are called Juggernauts and the yellow naked ones that explode into poison gas are called Bloaters. As for Juggernauts it's obvious why they're an issue, you can't run them over just like the weaker zombies and they also do a ton of damage to your vehicle if you ram them or they attack you. So get your vehicle a safe distance away and then shoot him.
Bloaters you don't really have to avoid, if you happen to hit one, just jump out of your vehicle and quickly use your medicine if you get low on health. Just something to be aware of.
Using the above method you can clear the 500 kills in about 20 minutes.
You can also try this method in the video below posted in Klaash
's solution from two years ago.
Feel free to read either solution that was posted two years ago: