That would be my guess as well.Oh, of course that's how it works. Power Attack has been noted to get extra dice at certain levels, and I bet that's proficiency instead. So one more at master, and another at legendary.
I expect certain combat feats will likewise upgrade with weapon proficiency.
Personally I would like to see Power Attack just be sloted into an action you can take once your attack bonus gets high enough. Or how ever they want it to work.
But Power Attack shouldn't be as much as a Gateway/tax is used to be so who knows. But if it's still basically required to play Martial..., just put it into the class/combat options from the word go.
Picking Power Attack is just officially declaring that you are playing a two-handed weapon fighter. It doesn't compete with the options designed for other playstyles, which is intended and what we want.