we have very low Action point

so how can a person earn 7000 piont in a day ???

he wasnt also the best killer !

Just because you have low AP, does not mean everyone got it.
He could have saved the AP, and could very well have hundreds.

You do not only gain Rep by killing people, but also by doing looms, selling bullets etc.
So a person with hundreds of AP, could make hundreds of Looms, and depending on what gear and STI he got, he can earn 100k-200k per loom hit.

As you can see in the top 10, not everyone got alot of kills.
Same goal (to get high rep) but with different tactics.

Hope that answered your question