Is it a child’s leg?

The most important question to ask when evaluating end-to-end encryption: who manages the keys?

If Facebook manages all of the keys and is responsible for telling which public key belongs to who, then of course Facebook can read every message.

“Sideloading” is their term, invented to make it sound like something it is not. We should not use this word. The correct word is “installing”.

You don’t “sideload” on Windows when you install software outside of the Microsoft Store™️. There is no real difference or distinction with software on phones, so there is no need for a special word.