Many startups are C corporations which typically do not have a sole "owner" i.e. I'm an owner of BackType, I'm also an owner of Google if I buy their stock, etc.

Essentially, it's up to the immigration offer to decide whether a certain level of equity disqualifies you from receiving TN status. Much more scrutiny is applied to the actual proposed employment, your responsibilities, whether these fit in the TN status categories, etc.

The TN is obviously not intended for self employment, but that becomes subjective once you've grown beyond a company of size one.