That argument requires one to assume that Canada might withhold shipments of steel to the United States at some critical moment. Otherwise, it doesn't matter whether steel comes from the US or Canada. As long as Canada doesn't shut down shipments, the United States' access to steel is secure, even if 100% of it comes from Canada.
The argument that Canada might cut off US access to steel is not plausible. The fact that the Trump administration is using the phrase "national security" to try to wring economic and political concessions out of Canada is transparent.
The argument that Canada might cut off US access to steel is not plausible. The fact that the Trump administration is using the phrase "national security" to try to wring economic and political concessions out of Canada is transparent.