If that would be the case what is the point of the article? Of course Google Play Protect shouldn't interfere with an side-loaded app. One major reason for side-loading (after giving explicit consent and ignoring all the warnings associated) is to allow applications Google wouldn't approve.
Google Play Protect also warned on unknown sideloaded apps (and requested an upload for a scan) when I tried it ~half a year ago. Documentation[0] implies this is still the case
> It checks your device for potentially harmful apps from other sources. These harmful apps are sometimes called malware.
> If you choose to install apps from unknown sources outside of the Google Play Store, turning on the “Improve harmful app detection” setting will allow Google Play Protect to send unknown apps to Google to protect you from harmful apps.
Schlaefer|5 years ago
david_allison|5 years ago
> It checks your device for potentially harmful apps from other sources. These harmful apps are sometimes called malware.
> If you choose to install apps from unknown sources outside of the Google Play Store, turning on the “Improve harmful app detection” setting will allow Google Play Protect to send unknown apps to Google to protect you from harmful apps.
[0] https://support.google.com/googleplay/answer/2812853?hl=en