Without giving credence to colonizers' fears about being on the receiving end of colonial violence, the thought that the US would be demoralized fighting Canadians is so out of touch. The US military bros think Canadians are woke transgender socialists in need of US-style liberation.
Not to mention they frequently engage in violence on the local populace around military installations as it is. Hell, they poisoned an entire Canadian military base with Agents Orange, Purple, and White and didn't bat an eye. (https://www.canada.ca/en/department-national-defence/corporate/reports-publications/agent-orange.html)
If the US military was demoralized by fighting people "like them," they wouldn't be known for the violence they inflict on their neighbours, their families, and their fellow military members.