It's not a back door to anything. It's competition for the US and Japanese automotive manufacturers who are protected by the existing tariffs.
With NAFTA/USMCA it 100% is a backdoor. China couldn't give a rats ass about Canada the purpose of these proposed trade deal with Canada is to bypass USA tariffs into the USA market.
It's multiple things. Yes, the automotive manufacturers matter not just for business sense, but because manufacturing base is important to be able to leverage in case of a war. Manufacturing lines played key roles in WW2.
In addition to that, since we're on the car angle, Chinese EVs are basically just privacy nightmares. I mean, all cars are at this point, but that's why we definitely don't want Chinese ones coming across the Canadian border and ending up all over the place.
In the end there are in fact legitimate national security concerns that the tariffs address and Canada risks weakening those. So, that is the actual answer to why.