I hate that website. It is sooo unhelpful.
Have you had the "pleasure" of trying to speak to a human being in Swansea? You have to wade through multi-level menus and then sit on hold for 15 minutes until somebody is available. And as if that's not bad enough, they now expect everyone to call on an 0870 number which is going to cost most people considerably more than a normal long-distance call to 01792. I gave up contacting DVLA by telephone a long time ago -- It's not worth the trouble, and you can never be sure that anything you're told verbally is correct anyway. You can speak to a different clerk a week later and be told something quite contradictory. Get it in writing!
Yes, it is different for US. The reason they don't recognise US licences is that each licence is issued by the state in which it was earned and they say they can't keep track of all the different regulations.
For an organization which thrives on thousands of petty regulations, you wouldn't think it would be that much of a problem. They have to keep track of the now 25 different EU countries' different rules.
Last time I read anything about it, I think France had exchange agreements with two or three states. (Don't quote me on this, but I think one was South Carolina; I can't remember the others.) So if you had a license from one of those states, you could get a French license, then DVLA would be forced to exchange a French license for a U.K. one under EU rules.
Re Canadian licenses, surely DVLA has already had to deal with 12 different provincial/territorial systems for exchanges in the past? (Now 13 with Nunavut.)
An old friend of the family actually had problems some years ago. She moved out to Ontario in the late 1940s and has lived there ever since, but has never taken Canadian citizenship and has never held a U.K. driver's license. Apparently she was told that she was not allowed to drive on her Canadian license while visiting the U.K. -- Something about the 12-month allowance for foreign licenses not applying to British citizens.
The whole system is so full of inconsistencies, contradictions, and rules which are unfair and make no sense that it's ridiculous.