Every post I’ve read claims NetNewsWire stores its data here:

~/Library/Application Support/NetNewsWire/

It does not. Or at least, the latest version installed from homebrew-cask does not. Instead, it resides in a Mac sandbox here:

~/Library/Containers/com.blackpixel.netnewswire/

Putting here for reference, in case you were going crazy like me. Perhaps its to prepare for its sale on the App Store?