I think the blog is pretty spot-on. These are most likely Native American carvings that happen to look runic, although there's also the possibility that they were carved fairly recently in order to mimic runes.

There was a bit of a rune-fever in the late 1800s and early 1900s. This was a time when American nationalism was on the rise, and it was tied to the idea that all 'real' Americans were White Anglo-Saxon Protestants (WASPs). A lot of Americans were unhappy that America had been discovered by a Mediterranean Catholic, and they wanted to downplay the fact that the oldest city in the US wasn't just founded by Mediterranean Catholics, it was even named after an African Catholic and a 'Father of the Church' (Saint Augustine, FL; actually, Augustine usually gets a pass from Protestant theologians, thanks to his emphasis on grace, and most people who get tied up about race aren't insightful enough to question the race of major figures in history).

These cultural biases made alternative stories attractive, and there happened to be an influx of Scandinavian immigrants at this time, promoting one compelling story: America had first been discovered and settled by real 'white folks' during the Viking Age, and although everybody in Europe was technically a Catholic at the time, at least these folks came from sensible countries that firmly embraced Protestantism as soon as they got the chance. So Lief Ericsson became a household name, and there was a bit of a scramble to find evidence of Viking-Age settlement, both in New England and among the Scandinavians of the Midwest.

I'm not sure when someone first suggested that Vikings had etched runes into the particular rocks you reference, but there were a few sites that attracted attention in Maine. I suspect that either this site is one of them, or else those other sites with supposed evidence of Viking presence helped someone else make a connection between your rock and Viking rock art / runestones.

As a final note, we do of course know that people from the 'Viking diaspora' did travel to North America during the Viking Age. We have two sagas, and the general outlines agree, even if they're different in the details. And we have the beautiful site at L'Anse aux Meadows in Newfoundland, where four distinctly Norse buildings have been excavated. This looks more like a work camp than a settlement—there's no evidence that they brought women or livestock—and there's no evidence that the Greenlandic Norse ever attempted to permanently colonize North America. They did explore at least as far south as the St Lawrence estuary, where somebody picked up a butternut burl, later dropping it at L'Anse aux Meadows for a future archaeologist to find. But these certainly were not people who traveled across North America, leaving runestones in their wake. Your carvings are natural, Native American, or something modern.