These are super cool!
But is it super cool to pre-load pages flash style? (I'm not saying it isn't.)
I'm saying it certainly isn't! :)
The overlay appear-disappear (even without the in-between spinner animation) takes something like 1.5 seconds. I would certainly expect new content to appear way faster than that.
(Even if the page that I'm waiting to load would have a huge full-screen image, any reasonably fast internet connection should be able to load that faster than 1.5 seconds; and if it's a video, you can/should certainly show a static preview before the entire thing loads.)
And if preloading the page would take — say — 2 seconds, having such a jarring animation for such a short time will very likely make it appear to take longer than it does.
I'm having a hard time thinking of a single scenario where such full-page "modal" animations are useful. Even if there was something like a "generate complicated report" action that takes 10 seconds, an inline "processing…" indicator would certainly be better and more informative.
Such animation demos are certainly very useful css/js excercises, but nobody seems to promote them as "cool tech demos" that they usually are.
The inspiration page linked from the article is http://www.nizuka.fr/. The homepage is 850KB in size, and seems to load the background image 3 times, in 2 different sizes. Maybe they need the animation to make that page size appear more "responsive", but the time would certainly be much better spent optimizing the images and loading logic a bit. Instead they have a "Commence loading" progress bar, and then a nifty folding animation after that.
It's certainly useful to add animation to make things appear to be even faster than they are, but the first step should be actually making things fast, rather than trying to "hide" the unnecessary slowness.