I am an engineer and not a legal expert but this is what I get from some portions of Treaty:
Treaty 9 does not say they own the land covered by the treaty. What they 'Own' is the Reserve Land but even that they can not sell it but only the government can sell it to the benefit of a native community on that reserve. The remainder of the land they have only usage of.
Also it states clearly that government may appropriate even reserve land for needed purposes such as roads, railways and even mining, with the provision that if appropriated compensation to the band be paid.
They can try to go outside the bounds of treaty but if it comes to push and shoves, it ends up in court, the treaty governs. That is why the natives lost in the recent Supreme Court ruling.