Canada was still subject to the legal authority of the UK parliament until 1982, and was certainly considered fully part of the British Empire until WWI

Canada was essentially independent since WWI.

I think the article was discussing actual facts-on-the-ground empires, rather than polite legal fictions invented to spare the UK from having to face their loss of global status.

They lost it to home rule only after it took less than a month to travel there with steam ships, right?

The point was they couldn't hold the Canadian colony. Britain didn't give home-rule to Canada because of their good-will. American belligerence towards European presence in the New World, and their rise as a major military and economic power, made holding Canada practically impossible by the mid 1800s (for example, it was untannable for Britain to maintain a large military presence in Canada anymore). Britain fully expected Canada to be annexed. Without the civil war and reconstruction preoccupying Americans during the critical early and fledgling years of the Canadian Dominion, I don't think Canada would have survived as a nation.

In another thread, I also made an argument for another major mitigating factor with respect to Americas, namely, the native populations were wiped out by old-world diseases which prevented local rebellions from succeeding and allowed Europeans to establish large population centers.

But the article literally argues that the British loss of the American colonies in 1776 is an example of how an empire can’t retain territory at a month’s remove.

To which the existence of British colonies in Canada after 1776 is kind of a direct rejoinder, right? The eventual loss of those colonies is irrelevant to the point that you can’t explain the American revolution as being an inevitable consequence of remoteness.

An empire can't retain hostile and unpacified territory beyond a certain limit - which may be set by time, communications, access to local resources and imported logistics, other factors, and all of the above.

If the territory is fully pacified with no significant resistance and/or it's run by dedicated loyalists with plentiful local resources the limit doesn't apply.

Basically an empire can only retain territory against an active threat or resistance within certain limits. If there is no active threat, or the threat is too minor to be a concern, the territory can be considered stable and fully colonised.

Aha. So the rule is: empires can only hold territory that they are able to hold. Got it. Useful predictive principle.

And also after communication time dropped from 2 weeks to 2 minutes (telegraph, 1866).

wasn't the war of 1812 partly/mostly driven by a US desire to annex canada?

The American state by the mid-1800s was not that same as the one in the early 1800s. America grew into a major economic and military power by then, the point where it was untenable for Britain to maintain direct control over Canada, or to even maintain a major military presence there. The British were ostensibly kicked out of North America by America.