> All currencies are manipulated. It's a meaningless statement.

That's probably (mostly) true today, but it wasn't always the case and it doesn't have to be that way. More importantly, just because most countries manipulate their currencies doesn't mean its meaningless to take issue with the practice or to notice when one country does it more aggressively than others.

Okay, those are good points, but I don't think the statement is meaningless. China, as I think most understand, deliberately pumped RMB into the market in a manner that made its resources available to foreign markets by trading a capital account deficit for a trade surplus. It's literally the first thing discussed in the RMB wikipedia page: https://en.wikipedia.org/wiki/Renminbi.

Currency debasement is more monetary policy than fiscal policy I think. Either way I didn't read the GP comment as saying China can't do that, just that doing so is what gave them an edge long enough to gobble up a lot of our trade.

The same reason a sovereign state can't act unilaterally in any domain -- diplomacy. If your actions affect another (or even just inspire interest) then the other party may make decisions which affect your country.