It's like saying that Belgium and the Netherlands, or Spain and Portugal, or Germany and Switzerland are one historical quirk away from being the same countries.
Maybe the US should be part of Canada?
And Europe _absolutely_ should unite under a single government instead of this pseudo national semi-single-currency/market with a vague poorly representative European government designed mostly just to dance around the fact that these small states are stuck on archaic nationalist ideas and can't get along with a unified purpose. The world needs the strength a unified Europe could provide to counteract Russian aggression, the growth of Chinese power, and the crumbling cornerstone of world order the US is going through.
Being offended is strange.
I think you could say this about any of those countries, although Switzerland's mountainous location means that it would always resist being part of a larger polity.
It's definitely possible to intepret this the way Russia speaks about Ukraine - "They shouldn't even be a country *except for a historical quirk", but a charitable interpretation would be more along the lines of "things could have gone slightly differently and we'd be countrymen, but instead we brothers from a different mother (country)".
I mean that's a bit of an exaggeration. Canadians are basically Americans for all practical purpose to the degree you can barely tell them apart. It doesn't help that Canada has lacked any real national identity other then listing the few minor differences between it and the USA for decades.
Why does this even mean? Does national identity even really matter? It's like saying Californians are basically Texans for all practical purpose. Men and women are pretty similar. To suggest that they're so similar they may as well be the same is absolutely condescending.
As a Canadian, why would it be condescending to suggest that at some point in the distant past, Canada and the U.S. could have been a single country had history played out slightly differently? There is nothing offensive about it, if anything the fact that it's a claim about a historical matter only highlights how the two countries have evolved separately and independently.
Furthermore your other points are kind of bizzare. Spain and Portugal could absolutely have been a single country, and in fact they were under the Iberian Union. There are numerous other instances where the two countries came close to unifying.
The historical possibility of a unified Belgium and the Netherlands is even stronger since those two countries had been unified twice.
Germany and Switzerland however is a long shot, but at any rate I don't think anyone from Belgium, the Netherlands, Spain or Portugal would take offense or find it condescending that some historical event could have gone differently and reshaped all of Europe... taking offense to that suggestion as a Canadian, even during these times seems overly insecure and I don't think it's a sentiment shared by most of us.
Wars were fought. People died, generations were involved in discourse about national identity and where borders should be drawn.
The US and Canada were both at one point British properties, so by some definitions, we also used to be unified. Then we weren't.
Is it insecure? Maybe. The reality is that in a shooting war, we wouldn't last very long against the US, in all likelihood. Under these conditions, the least I can do is to push back against rhetoric that undermines our legitimacy as our own country.
Essentially we are even closer than many people think in terms of history, but Canadian identity was seeded from the beginning with the idea of rejecting being "American". We are indeed your closest brothers and sisters because of history, but it's no quirk at all that we're separate -- it's the entire reason we stayed separate at all.
You can also see the reverse play out -- what would become Alberta was settled by large numbers of American colonists moving to Canada, and to this day you can see the cultural impact of that in the politics and world view from the region.
It is not a surprise that region can't find anyone else (in the rest of the economic zone over which it claims dominion) willing to die for its interests, especially when their interests have been revealed to be nothing but "loot the rest of the nation".