You are incorrectly assuming that "treaties" can only occur between nation states. "Treating" with someone is is just an archaic synonym for "bargaining"  or "negotiating"  and any bargan or agreement, private or public, can be characterized as a "treaty". In other words there can be treaties between states but they are not limited to only inter- state situations. The Supreme Court has already held that Canada's treaties with aboriginal collectives are not international agreements- which is another reason UNDRIP should not be applied in Canada.