Ah, but indigenous groups are a whole different matter.
The SCC has ruled that in a lot of development on traditional First Nations land there is a "duty to consult". That seems to be different than needing the formal consent of the FN, but this area of law is still evolving. In these cases a "comfort letter" doesn't equal formal agreement, but probably is good enough to satisfy the duty to consult.
That doesn't take away from the fact though that in common law an "agreement to agree" is not legally binding. A letter from the NCC stating "we hope to come to an agreement with the Ottawa Senators regarding the development of a new hockey arena at Labretton flats" is almost meaningless at law.