The statement from the linked page simply means that green card holders can return to the US across the US-Canada land border. I don't really understand what this has to do with "country (restricted to travel due to COVID-19)" in your title. Green card holders can currently return to the US from any country, no matter what countries they have been to. This includes green card holders who have been to any part of China, Europe, Iran, or Brazil recently; there is no restriction on their entry to the US. All US entry bans so far have exempted green card holders. (See the entry bans on people who have been to Mainland China, Iran, the Schengen Area, the UK and Ireland, and Brazil, as well as the so-called "ban on immigrants".) So I am not really sure what you are trying to ask.
If you are asking whether a green card holder can go through Canada to circumvent some supposed restriction on entry to the US, the premise of that is false, as there is no restriction on any green card holder's entry to the US, as I mentioned above.
If you are trying to ask whether you can take an itinerary back to the US that goes through Canada, because even though you can go directly back to the US, the one through Canada is cheaper or you can only find flights that go through Canada or something, people transiting Canada are exempt from the ban on foreign nationals entering Canada who have been outside US and Canada in the last 14 days:
The following people may travel to Canada for an essential
(non-discretionary) purpose from a country other than the U.S.:
- transiting passengers (must remain in a Canadian airport to complete their connection)