"The Americans" are a couple of men traveling in the same region as the narrator and her friends. Although the narrator assumes they are Americans, she turns out to be wrong; they are actually Canadians. In fact, the "Americans" mistook the narrator and her group for Americans:
"Say, what part of the States are you all from? It's hard to tell, from your accent. Fred and me guessed Ohio."
"We're not from the States," I said, annoyed that he'd mistaken me for one of them.
"No kidding?" His face lit up, he'd seen a real native. "You from here? "
"Yes," I said. "We all are."
"So are we," said the back one unexpectedly.
The front one held out his hand, though five feet of water separated us. "I'm from Sarnia and Fred here, my brother-in-law, is from Toronto. We thought you were Yanks, with the hair and all" (15.23-28).
Regardless of all that, the narrator continues to refer to them as Americans because of her belief that they killed a heron for sport.