Interesting. That was not the case (or not the legal advice) twelve years ago when I went through all this. I guess it is a stretch to suppose that things would not have changed but I thought this was a fundamental characteristic of the TN visa.
How does one start this process? I work at a YC company fwiw on a TN visa.
I would have an attorney evaluate your background - through review of a resume and a follow-up call - to see what you might qualify for.