I mean if you're investing in the USD, you're not going to be investing in the currency itself. You're going to be investing in bonds that are supporting the government that is backing the USD, the US government would be this entity. Those have pretty low returns, though if you need safety or protection against inflation (TIPS bonds) this could be the best option for you.
I wouldn't though, that's just me in my situation though.
I guess he means buying usd not investing in it.
By buying bonds you are investing in the country and it's government. You rely on it to make the right decisions. If they for instance go to war and lose, your bonds and the currency that you bought will be worthless.
If you buy the currency you're in a way investing in it too. That's what forex traders are doing. Buying a currency in hope that it will gain value in relation to other currencies.
So, bonds are bonds, currencies are currencies.