It's sad that Tuta has fallen for Big Tech newspeak. Once upon a time, we installed stuff on our devices. Calling it side-something makes it seem frowned upon, which plays right into Google's hand.
Sideloading is a type of installing defined by the source of the file(s) being installed. Google will still allow installing from the Play store. The distinction is valid.
Guess it's a good thing that Android can't install updates via Ethernet, but I only connect my phone via Ethernet. My next phone is going to be a Linux phone, though, I guess.