Work performed physically in Canada is not US employment and does not require TN status, but it changes the record: the applicant becomes an existing employee whose US entry must be as a TN professional, not a visitor. Keep the pre-entry work documented as Canadian employment and file the TN before any US work begins.
Sequence the start date around the admission
Hypothetical example: the distributor asks the West Kelowna CPA to begin month-end close remotely in June and to relocate in September. Performing that work in Canada is lawful from the US immigration perspective, but it creates a paper trail in which the applicant is already on payroll before the TN request. That is not fatal, provided the file explains it plainly and the applicant does not cross the border to attend meetings or training as a visitor while employed.
A person employed by a US company who enters to do that company's work is working, and visitor admission does not cover it. The clean sequence is to sign the offer, gather the duties statement and credentials, apply for TN admission at the port of entry with a start date on or after the entry, and only then perform work inside the United States. If the employer prefers an I-129 petition, the applicant waits in Canada for the approval and enters afterwards.