Basically, you've got it right.
I will summarize what I said. No, we can't have a preferential agreement with the States. That's the rule. When we enter into a free trade agreement, however, we can include an exception. In NAFTA, there was a chapter on government procurement that governed our bilateral relations. CUSMA doesn't have one. Canada could not have a preferential relationship with the United States unless we added a chapter to CUSMA on government procurement or reached a side agreement that would be included in CUSMA.
Right now, we can't invoke any preferential relationship with the United States on government procurement that would justify their granting us a waiver. If they grant us a waiver, they must grant it to all other members of the Public Procurement Authority.