Probably means something got copyright claimed in the video. There shouldn't be any adverts because they're funded by patreon.
It's not because they are funded by patreon that they can't choose to show ads. I know Jim Sterling doesn't do this because he's funded on patreon, but it is not a rule.
I did see an ad yesterday as well on the banner saga 2 review, so I think they just enabled ads (I don't mind btw, I am used to it because of other channels anyway).
I didn't notice it before yesterday either, but I usually watch youtube on PS4 and I watched the review on my tablet yesterday. Ads are not always shown (on the console apps at least).