Your citation doesn’t reference the Senecas, only “the Oneida, Onondaga, and Cayuga Nations.” And at least two of those tribes (Oneida and Cayuga) have been found to have lost their reservations. See the Sherrill decision and the Court of Appeals decision Cayuga Indian Nation v Pataki.
My citation didn’t, but the treaty in its entirety does, and outlines the land by geographic boundary for the tribes. Plus it’s hosted on the Seneca Nation of Indians website.