"But it's another sign that competing with Apple, on its own platform, is increasingly difficult."
No it isn't. It's a sign that this app has not kept up with the competition. Because:
"Castro is a podcast app that's been around for a long, long time. I used it years ago before switching to other apps like Pocket Casts and now, Overcast."
Neither of which are Apple products. Seems like they are doing OK. This is also a very crowded field.
Phone developers generally love Apple. They make around 75% of their money from Apple apps, even though there are FAR more Android phones in use world wide.
Because the vast majority of Android phones in use are very cheap phones and the owners aren't interested in spending money on apps.