Commodities are fungible. If Canada is making steel, which they are, it can export the entire output to the US, and then use Chinese steel for their domestic needs.
Trump wouldn’t care if Canada imported steel and used it domestically. He just doesn’t want China steel or other countries products coming into the US bypassing their own agreements because he was giving Canada favourable nation status.
Canada finds it cheaper to export raw timber to China for processing, then re-importing it. At the expense of their own mill industry. Incredibly myopic.
They want to keep their steel and export the crappy Chinese steel and call it Canadian.