I see, wow. That's such a frustrating lack of clarity on Google's part and (consequently?) those responding to the blog post...

As far as I now, historically, "sideloading" has always meant "installing from some mechanism other than the Play Store", and everyone has been referring to adb-based installations as "sideloading" as long as I can remember (example [1]). If Google or others don't call using adb sideloading, then I have no idea what they would call it, and I'm thoroughly confused.

[1] https://www.xda-developers.com/how-to-sideload-apps-android-...