Sounds like Scott Moe is being an ass. I'm skeptical he has a legal leg to stand on. I suspect this is an even bigger long-shot than Horgan's pipeline challenge.
More importantly, from the Saskatchewan point of view, is that the federal government's willingness to approve and support pipeline development is contingent upon provincial cooperation on environmental initiatives. Trudeau made clear when he approved the Kinder-Morgan expansion that it would not have been approved without Notley's initiatives on carbon tax, phasing out coal, and capping emissions from the oil-sands. This kind of compromise works better than the alternative did. It gives "wins" to both the economic development people and the environment people. But as Moe and his predecessor have been riding on Ms Notley's coat-tails as far as getting the pipeline approved and built, perhaps they feel that they don't have to give up anything themselves.
-k