You can develop and install via adb, but you can't just tell the package manager to install an APK you downloaded on your phone. Maybe attestation makes sense to allow Amazon App Store or Epic Games Store to be installed without a warning and to allow companies like Spotify to distribute their apps themselves from their websites without using Google Play Store and without a warning. What's wrong is preventing people from installing apps that haven't been attested by Google straight from their phone, even with a warning.
I get that requiring attestation for downloaded apps is wrong too, it's just this website says "it will no longer be possible to develop apps for the Android platform without first registering centrally with Google" which seems incorrect from what you're saying.
Edit: Oh I get it, "develop for the platform" means develop and distribute. Maybe it's just me, but seems like an important difference.
I think it is and it doesn't just end there. It's develop and distribute binaries.
Everyone is still free to develop and distribute source code.