Beyond the U.S. law, there’s also a fairly interesting issue of how international law regards this sort of treaty-making. As I’ve written previously, international law imposes two conditions on treaty-making by a sub-national actor: (1) explicit treaty-making authority from the State of which it is a component part (whether ex-ante or ex-post); and (2) the consent of potential treaty-partners to the sub-national actors’ participation in the treaty itself. Here, it seems we have a willing group of treaty partners, so the treaty seems OK on the second element (that is, assuming the Canadian First Nations are themselves authorized to make treaties under Canadian law).
Legal analysis on the import of the treaty from the international law scholars at Opinio Juris, here. An excerpt: