I don't believe you're reading that correctly. First of all the manual you reference is not the law, its the guidelines for an officer looking at you at the POE.
An intent to immigrate in the future that is in no way connected to the proposed immediate trip.
How I think a reasonable officer would read this: If you are coming in for TN and they ask you about you intent if you say after the next 3 years I am planning on moving back to Canada but in 10 years I want to try to immigrate, that shouldn't disqualify you for the immediate trip.
The trip is the duration of your current stay, not just the entry.
An intent to immigrate in the future that is in no way connected to the proposed immediate trip.
How I think a reasonable officer would read this: If you are coming in for TN and they ask you about you intent if you say after the next 3 years I am planning on moving back to Canada but in 10 years I want to try to immigrate, that shouldn't disqualify you for the immediate trip.
The trip is the duration of your current stay, not just the entry.