This is false. I say this as someone with a TN visa, which I use to visit my employer a few times a year.
The TN application process is onerous and takes weeks to confirm. The actual visa is indeed granted at the port of entry but can be denied - simply having the completed application doesn't guarantee anything. And you have to go through the complete application process every time you re-apply. There is no renewal application.
The best way to get it done is to work for BigCorp, Inc. whose team of lawyers will handle it for you. If you work for a smaller company or are a contractor/consultant, you'll have to hire a lawyer for a few grand. I know because I've been there too.
When I moved from Canada to the Bay Area, I accepted my job offer on a Friday evening, organized to get original transcripts from my university on Tuesday morning, FedEx'd them overnight to the law firm and they arrived on Wednesday. I received my TN-1 package (a thick, completely opaque set of forms, documents, letters and all sorts of other legal shenanigans) on Friday evening. I then bought my ticket to leave Canada the following Friday. All together it was 2 weeks from acceptance to arrival in the US.
Not every experience is the same, and no doubt it would have been far harder had my company not pipelined my immigration through a very prominent and large law firm.
That being said, the fact that I can be denied entry in the US at any time scares me. It scares my company. It's no secret it's expensive to hire, and even more expensive to hire new talent to replace talent that cannot enter the country. I say this as a young, educated professional with no criminal history or ties. I shouldn't have to worry about this.
takes weeks to confirm