If I browse a piece of software from play store and click ‘install’ it’s “installing” and if I do the very same with F-droid it’s suddenly “sideloading”. Fundamentally every language is just made up, but on this occasion the newly coined term is used to obfuscate things and attempting to paint things something they are not.

I can claim all day that grass is blue and sky is green, but no one will take me seriously. Same thing should happen with ‘sideloading’ vs 'installing. Or if you really insist, sideloading might be something like injecting code to a system in a way which is not normally possible, like how some rootkits for devices work. But ‘sideloading’ is very different from ‘installing’ and installing anything on a general purpose computer doesn’t include any particular tool (like play store). I can install things on my workstation with ‘apt-get install’ or from source via ‘make install’, but the end result is still that a piece of software was installed.