It is strange that it doesn't use any type of animated transition so it does look like a typical FOUC issue even though it probably isn't.

It is good design, now it looks like a mistake so people react and talk about it rather than just ignore it.

It's definitely not intentional. If it was intentional it would be animated, and there's no indication in the HTML that it's anything more than a FOUC.

If it was intentional shouldn't it have some kind of fade animation?