It is, because it's actually the term that defines the process of transferring files not from an external networked device - downloading - or to an external networked device - uploading - but between two local devices - sideloading.

It's over two decades old, you downloaded an mp3 from kazaa, and then sideloaded it to your player.

For android apps, I believe the term originates from the method of using ADB to directly write the app to the phone memory, the command of which is "adb sideload filename"