return ends a function, you would only go to the next line if it didnt return

Оставшаяся часть кода после return не будет выполнятся, программа выйдет из функции, поэтому это работает без написания else

Not sure which language you use but in JS it is not expecting an empty array.
This is the test:

var tests = [
    {
      "input": "A1",
      "expected": ["A2", "A3", "A4", "A5", "A6", "A7", "A8", "B1", 
    "B2", "C1", "C3", "D1", "D4", "E1", "E5", "F1", "F6", "G1", "G7", "H1", "H8"]
    },
    ...

return -1 is ours else

There's a difference between the normal if: else: statement in Python and this. This is specifically what is called a "ternary operator", and it's a different syntax for a different operation (although it does similar things.

https://www.geeksforgeeks.org/ternary-operator-in-python/

It's a bit of a shame that Python uses the same tokens if and else for both the conditional statement and the ternary operator - most languages don't, and it does make it confusing.