Yes incredible such nonsense could see print given the ease of fact checking today. As for the first American to win at Le Mans probably Luigi Chinetti (1949) would be a good candidate, depending on when his citizenship became final. If not he, then certainly Phil Hill won in 1958 and there is no argument about his nationality. This seems to confirm the comments about Phil Hill sort of falling through the cracks in some of these stories. Actually Hill would have won 3 times before Gurney managed to. Then there is Masten Gregory who won for Chinetti (NART) in 1965. I may have missed some others.
Chinetti was still travelling on an Italian passport when he flew into New York from Paris on December 24th 1949.