Carney was an INVESTMENT banker in the real world- quite a different breed of banker - at what is really the premier investment banking firm, Goldman Sachs, known more prominently in the investment world as just 'Goldman'. They play the long game, to make piles of generational wealth (not money, but wealth, there is a big difference between the two) and they play to win, to crush their opponents no matter what the cost. Goldman is a really a 'brotherhood' that exists around the world, across all western themed economies and they use their connections very very well.
Carney doesn't need Quebec to be playing nice if the decision is made to use Churchill as a eastern facing outlet to Europe (and the US?) for oil and/or LNG. Quebec doesn't need to be in the picture at all for this to work. Push the oil out of Churchill and out to Europe, to Saint John's and Irving's refinery (which it was just announced is going to have a few hundred million dollar refurbishment to it), Shipping oil to Saint John's will help reduce the need for foreign oil to the Maritimes.
And, as others have pointed out, Carney is moving at what is considered 'breakneck' speed in the political world. He fully realises that he needs to be seen as moving the peanut up the hill ASAP. He gets it, as do the majority of the Premiers. When you have an NDP Premier (who is Indigenous) in Manitoba saying 'yes' to an oil/gas pipeline through to northern Manitoba, through Indigenous lands, you have to come to the conclusion that the need for this cuts across existing party dogma and that it truly is a nation building (saving?) undertaking.