This won't affect the vast majority of our readers, and Google isn't going out of its way to further lock down Android as a platform at least, not with this. The entire point of the Advanced Protection Program is the extra security measures it imposes to enhance security, and this would be a very logical and consistent addition.
Based on that apparent inconsistency in the app's code, perhaps even this blocking of sideloading apps might be an optional setting, or this exemption could be in place for businesses and other organizations using the Advanced Protection Program that still require sideloading apps.
And again, before you get too worried about downloading apps from APK Mirror, by all current indications, this will be purely an opt-in change.