This is bad advice, the moment you apply for a green card you're non-compliant with dual intent of your current TN status.
The legal explanation for TN visas is the following: An intent to immigrate in the future that is in no way connected to the proposed immediate trip need not in itself result in a finding that the immediate trip is not temporary. Repeated renewal of a TN visa that leads to extended stay in the United States, may still be temporary, as long as there is no immediate intent to immigrate. (9 FAM 402.17-7)
TL;DR what matters is present intent at the time of application and every time you enter the US. It is not illegal if your intent changes after being here for a while.
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.