Yes, Overcast is iOS-only, though the developer does allow the iPad version to run on macOS and visionOS and will fix bugs. An Android version (or anything outside the Apple ecosystem, really) is explicitly not planned. He has mentioned that multiple times publicly.
There is nothing wrong to be iOS only but I believe OP's point is that an iOS app can't replace Google Postcasts.
My comment was meant as neither criticism nor support, only as clarification for those looking for alternatives and considering Overcast.