By "Hanlon's razor," he's implying the developers may just be stupid.

And can set an explicit size to prevent layout shift. This certainly feels intentional.

>And can set an explicit size to prevent layout shift

works great if you have tight coupling, but what if the guys in charge of the login page is on a different team than the marketing guys? What if the marketing guys want the ability to arbitrarily change ads without involving the login page team? I don't dispute that it could be made better, but it's very easy to build it badly, and there isn't much evidence that it's intentionally built badly.