There shouldnt be a difference but there is. Most iPhones require sideloading to get non-Apple App store apps.
The issue is for most people, if the government fucks around and gets Signal banned, Apple is their enforcer all the while harping about security and standards and other nonsense. Its horseshit because the lack of encrypted messengers like Signal makes people sitting ducks for surveillance and prosecution from a corrupt governement and Apple enforces their violent monopoly on deciding if the people using its products have safe access to quality, encrypted appz