I've been using the term side loading since the late 90s/early 2000s for installing software or files to a device via a transfer cable. And by the time Android came along, the early app development community was using the term to push the app to your device via ADB. And from there it's expanded from transfer cable push to download and install from an unmanaged 3rd party source on a mobile computing device.
So the term has existed in some form throughout the tech/power user community before modern mobile computing. Now did Apple and Google usurp the term? Ehhh, possibly? I've yet to encounter somebody that uses sideloading to mean something negative, but I'm sure there is a group out there that does. I'm not convinced that group is large enough for me to stop using language I started using nearly 30 years ago to mean something specific. "Why use more, less precise, words when this single term says it already?"
