The reference to 1821 is to the founding of McGill University, not to the Mohawk land claim. I checked the original borders of Kahnawake, the Mohawk reserve to the south of Montreal, and while they were larger in 1719, it didn't extend across the river. Unless there's something I'm missing, the Mohawk claim on Montreal itself goes much further back, as the Mohawk have long claimed the island as part of their territory. In 1727, Cadwallader Colden's The History of the Five Indian Nations, English-language history of the Haudenosaunee (the Iroquois Confederacy, rotinoshonni in the notice sent to McGill University) makes reference to this claim.

It all goes back to Hochelaga, the Iroquoian town on the island in 1534, when Cartier arrived during his second voyage. Between his second and third voyage, a war broke out between the the Iroquoian people of the St. Lawrence River and the Algonquin to the north (with the St. Lawrence Iroquois possibly weakened by an epidemic brought by Cartier). The Algonquin won, and the St. Lawrence Iroquois were pushed south and west. Some joined the Wendat (Huron) in southern Ontario, others - those from Hochelaga in particular - joined the Mohawk in eastern New York. They may have been an northern extension of the Mohawk; it's a bit unclear if if that claim is a retroactive application of their assimilation into the Mohawk or not. Regardless, the Mohawk from that point onward (if not before) regarded Montreal as part of their territory, even if they were momentarily run out of it. The establishment of Kahnawake in 1719 was a way to reassert Mohawk claim over the area, colonialist land titles aside.