I think it's probably meaning the 'itunes:type' tag, which is a new one. Apple's description of it is:
<itunes:type>
The type of show.
If your show is Serial you must use this tag.
Its values can be one of the following:
- Episodic (default). Specify episodic when episodes are intended to be consumed without any specific order. Apple Podcasts will present newest episodes first and display the publish date (required) of each episode. If organized into seasons, the newest season will be presented first - otherwise, episodes will be grouped by year published, newest first.
- For new subscribers, Apple Podcasts adds the newest, most recent episode in their Library.
- Serial. Specify serial when episodes are intended to be consumed in sequential order. Apple Podcasts will present the oldest episodes first and display the episode numbers (required) of each episode. If organized into seasons, the newest season will be presented first and <itunes:episode> numbers must be given for each episode.
- For new subscribers, Apple Podcasts adds the first episode to their Library, or the entire current season if using seasons.
(From https://help.apple.com/itc/podcasts_connect/#/itcb54353390)
Whether feedburner has heard of this I don't know: but refusing a feed on its absence seems a bit over the top, particularly as the implication of Apple's own documentation is that it's not required unless the show is a serial, which yours isn't. Incidentally your 'type' attribute in the 'enclosure' tag is correct and isn't likely to be an issue.
