How tf would anyone think that swiping a calculator screen will make the digits go away one at a time, and that this behavior would be organically discovered?
As with all design you can have the discussion if there are better alternatives, but it's not that crazy to design it like this considering most old physical calculators don't have a backspace button.
They have C and CE (or AC) buttons to either clear the last entry or everything, similar to the app. You could very well reason they are sticking to convention.
Also in most basic use of a calculator it's quite fine to input the last number again, it's probably often even more convenient than backspace.
This makes the backspace swipe feature an extra, non-essential feature, and it's probably ok to not be discovered organically. Most UI is full of hard to organically discover accelerators aimed at expert users. They are intentionally not visible, to keep the basics simple for novice users.
The right question is; can people use the calculator app without knowing about this feature? The answer is yes, as many people do without any problem.
Again, you could have a discussion if there should be a visible backspace button, but accelerators are an important part of any well designed UI. To answer your question: They don't think it will be discovered organically, it's by design.
