Why is my podcast validating correctly but getting rejected by iTunes?
Well, it looks like the problem may have been related to http vs https.

I found this interesting note on Apple's "Podcast best practices" page at https://help.apple.com/itc/podcasts_connect/#/itc2b3780e76

"Pick a reliable podcast host and be sure to host podcast episodes, artwork, and XML on an HTTPS-authenticated server. Podcast providers are reliant on the up-time, security, and speed of the podcast host distributing their RSS feed, podcast artwork, and episode files. For more information, see Podcast Partner Search."

So I added an SSL cert, cleared the cache, and resubmitted. This time, I got an email back saying, "Your podcast feed, [ https://www.loatoday.net/feed/mp3/ ] was successfully added and is now under review."

This is particularly weird because Roger Wilmut1 over at discussions.apple.com told me that Apple used to refuse https feeds. So apparently this is a change they instituted sometime in the past year or two. Maybe it is an outgrowth of the Apple vs FBI controversy over the terrorist's iPhone earlier this year? Who knows?

All I know is that by changing http to https I was able to get Apple to accept my submission for review.