I think I understand it pretty well, thanks.

Let's imagine all players start off on equal footing. Equal skill, equal starts, et cetera. Player 1 and player 2 are on different teams, and they both reach req level 4. They both have enough energy to use a requisition. HOWEVER. Player 1 has bought a bunch of req packs and has some legendary weapon available. Player 2 only has the base options because they have not bought any req packs or opened anything of use in the comparatively meager amount earned by playing arena.

In this case, player 1 has absolutely paid for an advantage over player 2.

I think it's pretty hard NOT to see that as pay-to-win.