Others have pointed out the relevant parts of RIC-3 and Treaty Series 1952 No. 7, though I suppose this does not answer your actual question, which can be paraphrased as:
Is being a US Permanent Resident sufficient for the requirement of
this treaty that licensed US operators be US citizens?
Strictly speaking, I think the answer is no, it is not sufficient.
This is because, even though you hold a "green card" you still must travel internationally under the passport of your home country. For the purpose of Canadian and US law, under which the Treaty was created, you are not a US citizen.
As other have pointed out (some saltier than others) it is unlikely you will run into any trouble, though. If you operate within the parameters given in RIC-3 no one will even care or notice. Since Canadian citizenship is not required for acquiring a Canadian license, and most hams near the border know we just share the air with only a few changes to operating requirements, it is unlikely that the letter of this law will ever be enforced.
The decision is yours, of course. It may be worthwhile seeing what the downside of being "caught" is, and if this would put your residency in jeopardy. I suspect there are few downsides, but I'm not the one taking the chance.
Anecdotally, the coursework for acquiring a Basic license in Canada never mentions citizenship in the section about operating internationally and in the US. This is probably because citizenship was a requirement for acquiring a license in the past, but this is no longer the case.
If the requirement has been relaxed, or if the legal language in the Treaty defers to modern licensing and operating requirements, though, I have not seen it. But, you know, IANAL, but someone might be able to find that in the publically available statutes.