I always ignore any terms that try to shrug off “sideloading” as a made up euphemism or anything because I think it applies to any system that offers a built in way to download and install software.

Apt on Debian and Ubuntu ships with repositories maintained by the community. But users can also choose to use other resources, including ones from off the internet entirely, or call dpkg with a file directly. So I would personally call making any changes to that to be “sideloading” in a way. The only difference is that you are allowed (and actively encouraged) to do so with Linux (as long as you have an understanding of what you are doing).