Decompiling is not the same as providing source code. When source code is compiled into a binary you lose eg the names of functions and variables, and there are tons of different ways to decompile a binary executable as there are always multiple ways of writing code that compiles into the same executable code. Decompiled code requires a lot of human attention to make it readable, maintainable, and easy to work with. You would not be able to get Maddy's original code from a decompiled Celeste as there will be so many places where you don't know what she named a variable, or how exactly she coded something. Just because you have the binary executable of a game doesn't mean that you know how the source code was written. Releasing the source code would tell us exactly how the game was actually written.