Canadian citizen, TN worker, commute daily from Canada: am I a US person for tax purposes?

Basically, you are a US person if you are a US citizen or "resident alien" for tax purposes. You are a resident alien if you pass the Substantial Presence Test or the Green Card Test. Since you don't have a green card, it's just the Substantial Presence Test.

In Publication 519, there is a section on regular commuters from Canada or Mexico. It says that for regular commuters from Canada or Mexico, you do not count the days where you commute to work in the US.

If you exclude the days where you commute to work in the US, you likely do not have enough days of presence in the US to pass the Substantial Presence Test, and therefore I believe you are not a resident alien, and not a US person.