I'm just not sure people have been referring to that method when saying 'sideloading' and Google didn't mention sideloading specifically there.
This is what they say in the quote this article is about:
"Does this mean sideloading is going away on Android?
Absolutely not. Sideloading is fundamental to Android and it is not going away. Our new developer identity requirements are designed to protect users and developers from bad actors, not to limit choice. We want to make sure that if you download an app, it’s truly from the developer it claims to be published from, regardless of where you get the app. Verified developers will have the same freedom to distribute their apps directly to users through sideloading or through any app store they prefer."
In this paragraph they don't mention ABD at all similar to how in your paragraph they don't mention sideloading.
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-...
TVs are an entirely different class of sideloading than Phones.