The tutorial is beatable while not really using the surf features at all. The backstep mechanic is so overpowered that any incline trivialises the entire level, allowing you to shoot into the sky and land wherever you want. The air-strafing allows complete preservation of momentum with zero effort. Feels less like a recreate of source/quake movement and more just a list of hardcoded surfing rules.
Maybe take a look at the open source surfing recreations out there for some tips, I know of multiple Unity ones which use C# just like Godot that you could probably take some inspiration from.