To expand on what ssjmarx said, Canada's claim to land within its borders comes from the treaties the First Nations signed with the Crown (stand in for the Queen). A lot of First Nations territory in BC never actually had treaties signed; Canada's push to claim the west was partly to prevent the US from claiming it first so they worked fast and loose. This results in a lot of unsigned land being lumped into the category of "Crown land" (land owned by the government) and the government writing out permits to whatever company wants to fuck up the environment because they say it's their land now. Canadian courts usually treat this as Crown land and serve injunctions to anyone "interfering" with resource exploitation even if they legally own the land.

In this case, the traditional hereditary chiefs of each tribe of the nation are opposed to the CGL project and are leading this resistance. However, the government and CGL have the support of the "elected chiefs", the chiefs created by the racist Indian Act that are voted in based on more """western""" style """democratic""" governments. Laughably, the province of BC signed onto UNDRIP, which afaik means they still need to heed to the hereditary chiefs here.

So there's not really a legal basis except for the fact that the Canadian government says there's a legal basis, which is business as usual for settler colonial states.