I would wonder what the proper thing to do in this case is anyhow. As a user I'd hope I'd not be asked with a random interface to repost when some background JS runs XHR asynchronous from my interaction. That's obviously not workable. In this case the user of the agent is more like the application code and XHR doesn't really have the API to support this sort of event AFAIK.

Regardless, that is a separate issue to what is being discussed, and the article does mention it; I'm sure it's not that it was forgotten. Standards documents aren't always perfect in predicting every possible scenario so there must always be room for practical implementation and I think IE has a very good argument to make in its favor for this specific case.