Is this an advertisement? If so, what type? It doesn't seem to appear in their advertising documentation, either (which states that overlay advertisements are 480x70).
This is a call to action overlay (which is different from an overlay ad). CTA overlays are available in advertisements (if a video ever has been used as a video ad, any CTA overlays that have been inserted during the campaing remain on it afterwards), for non-profit accounts and for twitch live notifications.