That's moving the goalposts. The point was that signing and distributing an app outside of the App Store does not require a paid developer account, not that it would play nice with Gatekeeper. You can still option-click and run the installer even if it is not notarized.
Note that I will concede that it is extremely stupid and greedy of Apple to require a paid account for notarizing apps, particularly free apps, because it is in their best interest if more applications would be notarized, and they are not particularly hurt for cash either. It's inexplicable TBO. But that doesn't mean you cannot install signed but non-notarized apps without a paid developer account.
Note that I will concede that it is extremely stupid and greedy of Apple to require a paid account for notarizing apps, particularly free apps, because it is in their best interest if more applications would be notarized, and they are not particularly hurt for cash either. It's inexplicable TBO. But that doesn't mean you cannot install signed but non-notarized apps without a paid developer account.