If you know you can do that, iOS won't tell you as a new user, yet you know what a back button will do in Android, also it's tricky with a case on the iPhone sometimes to swipe like that, and you need a case because the iPhone is too thin and slippery.
EDIT:: I should add I don't use the swipe as it is a pain to do for me with the official Apple leather case on, it has the sides that are higher than the phone height so it's not easy to get the swipe to work, so it makes it slow and cumbersome, smacking a back button is much quicker I find.
I do not find it tricky with a case. I use several cases and I have yet to find it hard to swipe back. I have an Apple silicone case, a Diztronic case, an Evutec Karbon SI case and non of them present a problem swiping back. Nor do I have a problem with force touch with a case to access to app switcher.
Apple is not going to add a back button. If anything, Apple will soon remove the home button. It is becoming unnecessary.