Great question, this is something I probably should have cleared up better in that thread.
Stepik requires us to provide exactly what to accept, and there is some flexibility in our ability to take different versions of the same answer (adding a space after your answer, some extraneous formatting, etc). However, in general, it has pretty basic functionality, so for fill-in-the-blank questions, even if your inputs would create a program that, when run would provide the correct output, Stepik isn’t testing the result of running your code - it’s testing the exact inputs with what we’ve entered as an acceptable answer.
This is more constraining than life as a programmer would normally be, but these additional constraints also help build good habits for when you’re just starting out. In general, if you follow the guidelines outlined throughout the course, your answers should naturally fit what we’re looking for, so I’m hopeful this won’t be too big of a problem to overcome.