

It’s not a “bullshit new term”, it’s three decades old and means transferring files locally from one device to another, instead of directly downloading or uploading from/to an external server.
The origin goes back to MP3.com and i-drive in late 90’s, but the most common sideloading people did was downloading music to their PC using services like iTunes, and transferring them to their mp3 players. As they did often with early PDA and smartphone apps, where the term for Android comes from - get the .apk on your computer, transfer it to your phone, and install it.
Sideloading.

It is still the same installation method, directly installing the .apk file, from way back when the term for Android usage was defined. So, kinda, but also kinda not. Also, if you do use ADB to do the install from a PC, the command is “ADB sideload filename” which will do the transfer and installation to the memory directly. Then it truly is sideloading as defined.
Android doesn’t use ROMs (Read-only Memory) any more either, because the filesystems are now writable. But Lineage etc are still called custom ROMs, because the end result hasn’t changed.