>No it's not. The term originated far before this debacle, and carries a meaningful distinction than just "installing". Specifically it means installing from a non-first party source
It's amazing how many confidently wrong people are springing up out of the wordwork to present revisionist history about the meaning of "install" like it's ancient wisdom. Pre-mobile computing treated "install" as neutral and primary and had no built in relation to centralized distribution. Sideloading as a term of art originally, in practice came into usage for transferring media to devices, and some cloud file hosts briefly used it to mean load a file to an online drive without downloading it to computer. It's usage was varied, irregular, and not at any threshold of popular acceptance for one meaning or another.
Windows, Dos, Linux, and online self-hosted services had no notion of "sideloading", or at least no usage of that vocabulary and did not use this notion of "install" that is now being retrospectively declared a longstanding historical norm. Even now, that's not a term used in Windows or Linux. Even Apple, who very much in practice utilize this controlled distribution model but even they don't use this sideloading/installing verbal distinction. In Apple's lexicon installing is neutral with respect to where an app comes from.
So it's staggering to see a specific term of art that deviates from historical precedent that only is used in an Android context and only relatively recently in the history of computing be referred to as if its observing a longstanding precedent across all of computing. It's nothing of the sort.