I would think that a Passport Power Index would include more than just where you can go visa-free. Things like availability of embassies and consular services would also factor in. Is your government going to intervene if you get into trouble abroad? If so, how effective are they at doing so? Can you buy property outside your own country? If so, how many countries allow it and are there any restrictions?
I think your link takes a very narrow definition into its rankings. I'm not sure how broad the index in the story is, but it seems to take into account more than just visa requirements.
It lists the US as a number 1 country there (with 174 visa free countries), I guess they omit it on the free list to gather email addresses.