“On skippable ads, the button appears after 5 seconds into playback, as always.”

They aren’t hiding the skip-button, they are hiding the not-being-able-to skip-button.

I guess the advantage for Google is that users can’t know whether they’ll be able to skip, so they might watch more of the ad with expectations that they might be able to skip it.

They are making it smaller so users don’t see it as easily.