Villain's flop call instead of c-bet usually indicates something like A-high with a BD draw, or maybe a small pair like 3x, 4x, or 5x that is seeking a cheap SD and doesn't want to bloat the pot OOP. I bet for equity denial/protection against his unpaired over-card type hands.

I make the idiot end of the straight, but it should be good most of the time vs the hands villain could have here. His c/r is really strange. He's saying he's got only 7x, but 75s is pretty much the only 7x combo he could have because 76s flopped the nuts and wouldn't triple check it (would he?), and 78s, 79s, and A7s aren't c/c'ing flop. I suppose it's possible that some bad players could play A2s this way, but I block that and they're unlikely to c/r it given this river anyway.