The claim about carbon cost of shipping has no numbers attached to it and is most likely completely deceptive. It doesn't account for the energy return on energy invested of tar sands being 3x-10x less than conventional oil, an EROEI of 5 versus 18-50:

https://en.m.wikipedia.org/wiki/Energy_return_on_investment

We do not need this oil in particular, there's more than enough other oil, so there's no foregone conclusion that we are going to extract these tar sands.

Further, the need for stopping this particular pipeline is that we are not transitioning quickly enough, and it's a bad capital allocation that hurts our ability to make the investments we do need.

The pipeline isn't being installed to lower the carbon load of bringing the oil to market, it's only there to reduce costs, which means that there will be greater amounts of that oil brought to market, versus cheaper oil from other sources that don't require as much energy intensity. The pipeline only exists for these particular oil investors to profit, instead of other ones in other parts of the world.