Since none of us alive walking this earth today, have a living memory of who and when the markings took place, I am not cemented in one particular theory. I do however, lean toward the idea the markings are far older and appear to have more in connection with the numerous other known Norse sites.
In the grand scheme of things what these landmarks tell me is this continent has throughout the course of time had many different peoples spending much time here, long before Columbus sat sail.
Ask yourself some day, why is it that we have almost no records of 16th century North American ventures ~ just a handful of Spanish attempts at settlement, yet by 1604, according to the Treaty of London ~ right inside the treaty itself ~ they're working out a codicil for ALASKA ~
Yup, it'd been discovered ~ not the gold of course, but IT and its furs.
All the Swedish government records were in a building on the site of the present royal palace, and it burned to the ground about 1697. The Swedish government is still rebuilding its history by accessing all private records, land records, Bible notations, materials held abroad, etc.
There are an incredible number of Swedish traces all over what is now the United States ~ and there's the trace of a survey sponsored by North America's owner ~ to wit ~ Spain.
The simplest solution is the Swedes surveyed and sought access to furs, while the Spanish did a bit of development (grist mills, mines, trails, animal husbandry) ~and they did it together ~ or in conjunction with Spain's commercial interests.
if you sit in front of De'isles map of 1717, or maybe 1709, ask yourself 'How did he know' ~ there are things on those maps that NO ONE KNEW but the chief cartographer for France knew. I believe his great grandfather was Coligny, and Coligny had an incredibly intense interest in finding a Protestant homeland in America.