It's understandable why BYC and the USCG Lt. who approved the BYC Mac race considered the US Canadian border as inviolate, especially in light of how the conditions on the race turned out. The concentrated number of boats sailed hard would have meant a high probablity of a few ending up in Canadian harbors.
However, transiting US boats are allowed to cross into Canadian waters while transiting to and from non Canadian ports...As long as they don't touch Canadian soil, or hangout in Canadian waters sightseeing etc.. Below is the Canadian guidance:
Information for foreign boaters
Unless you are exempt, you cannot currently enter Canadian waters (territorial sea and internal waters) or boundary waters for discretionary (non-essential) reasons. These reasons include:
Consult Entry restrictions for more information.
You may still navigate through international or Canadian waters while in transit directly from one place outside Canada to another place outside Canada, if the transit is:
by the most reasonable route
Transiting travellers may only make non-discretionary (essential) stops along the way, including to use facilities, refuel or for essential supplies.
You must follow social distancing practices and wear a non-medical mass or face covering during these stops.
You may stop and anchor out of ordinary navigation, particularly if it becomes dangerous to navigate at night or if the crew must rest before safely continuing your trip.
If you anchor to spend the night, you must quarantine on your boat. If this is not possible, you may quarantine at a hotel until you are ready to resume your trip.
Consult Quarantine requirement for more information.
If at any point a transiting vessel lands on Canadian soil, anchors, moors or comes alongside another vessel in Canadian waters, or if anyone onboard disembarks in Canada, the operator must report to the CBSA. All entry restrictions apply.
The CBSA and its law enforcement partners are actively monitoring Canadian waterways. If you fail to report to the CBSA, regardless if your purpose is non-discretionary (essential) such as to refuel, you may face severe penalties. Failure to report may also affect your immigration admissibility and ability to re-enter Canada in the future.