> the few listeners who do skip ads continue to remain engaged with the episode, rather than dropping off at the first sign of an interruption.
I'd bet that this is because fast-forwarding through podcast ads is pretty trivial. If podcast apps tried to do anything to make the midroll ads unskippable, you'd probably see a different pattern. (Here I'm thinking of things like online video ads and the TV networks' fight against the TiVo 30-second skip button.)
For my part, I often skip through ads that I know have no relevance to me, but listen to ones that might have relevance. It also helps that many podcasts bring their own humor to the ads rather than just reading the ad copy verbatim, so there's still some entertainment value even if I know I'm not going to buy what they're selling.