I don't know if adb sideload will fallback to a regular install, maybe it does as a convenience for people who don't read the docs? In any case, you said:

That’s the name of the command, though.

And I demonstrated that you're wrong by linking to the official man page (aka the docs) directly from the master branch of the public git repo. Here's the relevant part which describes the sideload command:

sideload OTAPACKAGE Sideload the given full OTA package OTAPACKAGE.