So Lex Luthor's power looks into PTs played before his ability triggered right? How does the wording convey it? How would the wording be different if it didn't look before him?

For reference:

At the start of the build phase, each opponent draws two cards. If he does, that opponent can't play more than one plot twist this turn.

Emphasis mine. :)

To expand the point, the wording as is just suggests that the opponents cannot play more than one plot twist that turn at all. If you've played one or more, that restriction doesn't go back in time, but it automatically goes online preventing more being played.

If they were to word it so it only restricted NEW plot twists from being played, they'd have had to make it a lot clunkier, or they'd have to restrict it like how Witch Woman's effect works only within a specific phase. The problem is that triggered effects (which would be the easiest way to make the post-effect-only restriction work) actually have to resolve. There would have to be some sort of odd floating modifier that's created and somehow turns on immediately. The game currently does not support that very well, so it'd be very unlikely to come up in the near future.