These are ads inserted directly into the stream by Megaphone. TWiT uses Megaphone to fill ad slots that don’t get filled with host read ads on some podcasts from time to time. (Clearly they would prefer to sell all ad slots for host reading at all times.) There is more info here in the forums (mostly about how to prevent ad blockers from failing to download the podcasts) if you’re interested, search for Megaphone.
One example post: TWIG using megaphone.fm