Ex-U.S. developed markets could be an interesting diversifier.

IMO (not worth much), it's not so much the current valuation level (e.g. current P/E, P/B) that's key, but how these levels are changing over some reasonable past stretch of time.
For example if one did a similar slice&dice exercise on the U.S. market, e.g. dividing it into "sectors", then some have always had a much higher P/E (or P/B) than others but this doesn't necessarily mean that a certain sector is more attractive, based on it's current valuation, relative to others.

But if the "ex-US developed countries" segment of the world market is becoming increasingly cheap on a value basis, while the U.S. segment is becoming increasingly expensive, then it makes that investment thesis more attractive.