POTUS needs to cite national security as is how he is able to unilaterally place those tariffs without congress. Furthermore natsec threats doesn't mean that a partner is not trustworthy. Australia is one of America's longest standing and most dependable ally. If the US were to use Australia as it's primary rare earth mineral supplier a perfectly valid argument could be made that in the event of a global war an adversary could disrupt trade between the US and Australia. Thus there would be a national security threat.
He has access to more intelligence then you and I. He must legitimately feel Canada is a threat to our security. Possibly even imminent invasion, or hostile action. He has the intelligence.
Someone who will persist in a racist lie of birtherism conspiracy nonsense for five years, has zero credibility. What this person "legitimately feels" is so totally out of scope because first his assertions of fact are frequently provably not reality based, second there is no ability to assess anyone's feelings. Why must he legitimately feel Canada is a threat? Why can't he be lying about it just to get what he wants, whatever that is?
