The official statement from the White House calls out increased fentanyl production in Canada as the reason - but doesn't say that it's the majority of imports, you're right. The twitter summary says "There is growing production of fentanyl in Canada, and enough fentanyl was seized at the northern border last fiscal year to kill 9.8 million Americans.", though, and that seems like a lot.
This is an excuse, as an "emergency" is (effectively) the only way that such tariffs can be applied without input from Congress at all.
It's also likely a lie, given the number of lies told by Trump and his supporters. The two groups which should not be believed on this are the DEA (whose entire raison d’être is predicated on making things far scarier than they are so they can increase their power — I believe they are behind the proven lie that "touching fentanyl can kill you", which it can't) and the Trump administration (because the volume of lies spewed daily would overwhelm the New York City sewer system).
A "super lab" that used "cartel" processes was discovered in Canada a few months ago with almost 100 million[0] lethal doses of Fentanyl. It was only 11 miles from the border. The group was linked to a Khalistani transnational crime network; the same group of people (Punjabis) have also taken over our trucking industry[1]. Our border is the longest undefended border in the world and Canada is now taking in more than 1.5 million people per year[2], not including the 5 million on 'temporary permits'[3], mostly from India and China[4] and mostly un-vetted[5]. They get H2B visas with their Canadian passports and can travel across the border with ease. 1 billion doses worth of precursor chemicals were found in the port of Vancouver over a period of just 3 months last year[6], all coming from China. Halting immigration and shutting off the precursor chemical supply from China via the port Vancouver is all my clown show of government had to do to avoid this trade war. Stopping the flow of these brutal new drugs will do more to save both our economies than any amount of 'USA owned' tar sand ever could.
(This is also the US's stated reason for tariffs on Canada)