The link https://www.cdc.gov/coronavirus/2019-ncov/travelers/testing-international-air-travelers.html is about test requirements for all air passengers arriving in the US. Not just for US citizens. But this is for air only.
There is no test requirement for land crossing in the US for eligible persons which includes US citizens/residents and essential travellers. God only knows what the rules will be on the US land crossings if/when they open it for Canadians.
Like I wrote before, one-sided border closure is nonsense. The official US word is that they will follow the recommendations of the US healthcare professionals but this is a poor excuse too, because in Canada there are fewer cases per capita and higher vaccination rate than in the US.