It's hardly "cheating" if these things are between nations in the WTO which - because the US is a willing participant - has ultimate rule over international trade. Any nation that joins the WTO gives up a lot of sovereignty over such matters. And once you are in, kinda like the UK and the EU - getting out is much harder than you like to think.
I rather think that Trump didn't know much if anything of how the WTO and trade tarrifs work. He didn't want to settle these things the long and slow way with negotiations and meetings and compromise - which may not get the result he wanted - for the US always to be the winner, whatever that means since I rather think it is a zero-sum game here so the harder we win some things the harder we lose others). He just wanted to be the showy "You're fired!" Trump his base likes because that works on TV (although we have later heard it was actually the producers who told him who to fire anyway and they were the ones with the money - Trump always follows the money, well the short-term money anyway).
The G6 combined has almost as much GDP, and far more people than the US. They are also not so dependent on the US as we like to think. A good deal of oil is being traded in Euros too - Russia and China would probably not lose any sleep about selling to them before they get to the point of renewable energy independence.
Besides if you look into the who dairy thing that Trump brought up it's a drop in the ocean compared to all the other trade, and he conveniently forgot to mention that our own dairy industry is subsidized and always asking the Federal government for money, in 2016 it wanted the FDA to buy $150M worth of cheese they overproduced due to subsidies and protectionism. Somehow everyone bitches like crazy about subsidies of the USPS or Amtrak, but not the dairy industry - which is mostly run by big-ag companies making a fortune anyway.
I see this all as more populist chest beating that may help a relatively small number of people but does little for most Americans who need help the most. I'm pretty sure if you look closely, as with the coal deregulation, you'll find an actively highly funded industry lobby bugging Trump to do save them from the evil Canadians - mostly to benefit some large very profitable dairy product companies. That's allowed him to rant and rave (and tweet) about it with much gusto as he did with North Korea. The jury is still far from in on whether that does any good. I rather think Kim is going to end up winner in that game of juvenile hystrionics and button size bragging.