iframe's are the devil, it's true.
But, even if they are deprecated browsers won't stop supporting them, they're too helpful in fixing bugs in IE that can't be solved any other way.
They don't give any reasons in their article why they've gone with iframe's, my guess is because it's a sand box and it's harder to break the rest of the page.
Less maintenance for them.
It's also a much simpler method and means people can actually do more than they could previously, as they have full control over what goes in there.