Each state has its own laxness when it comes to federal importation.
To be clear however, your car was "exemptable" from import only for one-year, based on the past practice of one-year duration of your visit on
TN status. Once that year is up, you would either have to leave or have it formally imported.
The fact that TNs can last less than a year allows you to drive it without importing it, but that allowance lasts only one year.
If the "paperwork" you are referring to is the NHTSA-compliance letter from the Cdn manufacturer, and a customs form, then you have formally imported the car (and should have done this when you brought the car down in the first place). Otherwise, you wll be relying on the same laxness for another DMV employee when you go to trade your car.
Just for others, I would question whether any insuarnce you had during the time your car was Cdn-plated had any validity at all, since your plates were likely invalid, which is grounds for insuarnce denial.