Your U.S. passport is your [return] travel document, until you get your PR Card. Your CoPR is the document that proves PR status when you encounter CBSA.
Don't stress!
But COPR is not a travel document, correct? I understand it should be easy for me to travel on US passport, but it's all pretty confusing when they want you to have PRTD.
When you say "they", do you mean CBSA or do you mean the airline?
It's widely know that a U.S. citizen traveling on a valid U.S. passport, doesn't really need a valid PR card (or PRTD) to return to Canada via a commercial carrier. This is because of a bilateral agreement between the airlines, apparently. Because of this, you should not volunteer that you are a PR of Canada.