I don't practice tax law but I did take income tax law in law school, and I recall that the test for residency isn't one of days per year or anything, but a wholistic test based on all the relevant factors. The fact that he's born and raised in Toronto, and plays in Toronto from October to May (with games on the road) is probably why the CRA is taking the position that he is a Canadian resident for tac purposes. I also assume he has a nice cottage in Ontario that he spends at least some time at during the summer.
Ya the # of days in the country isn't the end all be all but helps establish your tax residency. The reports suggest that the legal argument seems to come down to whether the signing bonus was an "inducement" to sign. I think the issue is how signing bonuses are taxed in Canada (it's the equivalent of salary). Reasonably speaking, an American with zero connections to Canada who gets a signing bonus on July 1st from a Canadian employer will be taxed by the IRS and the treaty provisions would apply and CRA taxes the signing bonus at 15%. I think that's clearly the spirt of the rule - that the signing bonus was an "inducement" to come to Canada. What if it applies to a Canadian who previously went to work in the States? The perception changes?
In terms of whether the signing bonus was an "inducement," I think the CRA has the better legal argument here. In the context of the NHL, the entirety of the signing bonus is counted as part of the salary cap. The signing bonus also isn't a one time thing but continues throughout the term of the contract. If Tavares decided not to report to camp/the team for non-medical reasons he would be suspended without pay. He certainly will not get the signing bonuses due to him in future years if he remains suspended. He would also very likely have to pay back the signing bonus he received.
In terms of the residency argument, it could be factual. If Tavares stayed in the United States and didn't return to Canada until beginning of camp, he clearly would have spent most of the year in the United States. If that was the case, there should be no argument that Tavares was a resident of the United States for tax purposes in 2018.
If we are to take a holistic approach to this, the signing bonus wasn't an inducement for Tavares to come play in Canada as he would only get the tax benefits in 2018 anyways. What the signing bonus is is that it is guaranteed money that Tavares gets from year to year as part of his renumeration.