My theory is that the inverted wingers is a result of losing David Silva.
Pep doesn't think Foden can operate in this role because he's too dynamic, and doesn't control the tempo well enough in the final third.
So his "work around" is to give KdB freedom to play higher and do damage in the final third, instead of his RCM role where he was typically deeper, and to use inverted wingers to rotationally play the "David Silva role", with Jesus doing the same he always does.
There is merit to the idea, if he believes David Silva is so necessary to his 433 style, then he picked a formation that "in theory" suits the forwards play styles, Mahrez, Bernardo, Sterling and Foden all like to play "inside" and not wide.
The problem is that
1) Sterling doesn't have any creativity for this role on the left, unlike Mahrez or Foden, and doesn't release (or pass) the ball quick enough.
2) Mahrez looks good in some games and not in others (need consistency), at the very least he is passing more and his creativity is top notch. I don't consider the current Mahrez much of a problem tbh.
3) KdB isn't scoring any of the chances he's getting.
2 Solutions from my perspective:
1) Play Foden in KdB's role as the 10, he can create and finish fairly well. Perhaps Mahrez here also. Then drop KdB into one of the 2 holding midfield positions, he can still create from deep better than anybody in the world.
Then play Ferran LW and Sterling ST (to rotate Jesus).
2) Just play Foden in the LCM 433 role and let him master it with game time. That way KdB is also playing deeper (perhaps better than as the 10 imo), and the wingers stay wide. The wingers may not prefer that, perhaps why the 4231 is being used, but if it works it works.