I don't know what is confusing you. It is always the case that CBSA officers at POE have the final decision to allow any foreigners to enter Canada. It is like this even without this travel restriction. But I don't see any reason if you have the authorization letter from Canadian government and your travel is essential, why would CBSA officers at POE deny your entrance to Canada.
When I read some who are on trv and with a letter being denied that's what worries me. If some are denied even with having a letter then it can happen to me too, maybe just cautious since I was denied once.