So it's all a lie to tank the US economy, then.

It's largely a lie, but Trump is not motivated to tank the US economy. He's going to, unfortunately; but because he's wrong, not because he wants that to happen.

Trump genuinely and sincerely believes that tariffs are almost a form of free money - a way to force other countries to give us lots of revenues, and that our country only benefits. Of course, that's not true. There's no such thing as a free lunch. Imposing tariffs cuts our country off from cheaper goods, both finished products and components of products that we manufacture ourselves - so we end up poorer when we prevent ourselves from buying those things. And then there's the inevitable retaliatory tariffs against our exports, which just heighten the pain.

Even under his (mistaken) economic theory, it's still a bad look for the U.S. to bully other countries into giving us free money just because we can - just because they're weaker than us. Hence, the claim that countries like Japan - which has one of the lowest tariff rates in the world - is allegedly hitting us with tariffs in the high double-digits. Which is simply not true. But we have to be the unfairly treated aggrieved party, rather than a country just trying use the fact that our economy is bigger than everyone else's to take advantage of them.