The whole "hidden bolt" thing was a bid to do away with an arm on the crank spider because there was a crank arm right there that could do double duty.

Crank manufacturers did some math & have since moved on with asymmetric 4 bolt designs when they realized that the 2 closer sides of the "X" were all that was needed during the power phase of each pedal stroke.

The hidden bolt thing really doesn't have much to do with power meters; You can get power meters in about any configuration you want. It was to copy Campagnolo who happened to apply some thought to their designs.