Did you try with the .xml URL? I haven’t tried adding a podcast so I’m not sure what would happen in that case.
Other than that I put the .xml feed through Cast Feed Validator and it looks like you’re missing the details that can be entered in your account settings: Micro.blog → Design → form at the top of the page.
If you have filled those details in and the .xml feed isn’t working, then either there’s a bug at play here or there is some way to get the .rss feed that isn’t immediately obvious.