I'm not sure, but it may be that you have to get each individual element, convert it to a dictionary, and then get the value once you're down to the correct data level. This forum post in 2018 has a snippet from a user that shows how they accessed JSON data in C# with Godot that may be helpful. Looking at that post, you would need something like this to get the format:
Godot.Collections.Dictionary datos = (Godot.Collections.Dictionary) JSONParseado.Result;
Godot.Collections.Dictionary editor_settings = (Godot.Collections.Dictionary) datos["editor_settings"];
Godot.Collections.Dictionary export_settings = (Godot.Collections.Dictionary) editor_settings ["export"]
string format = (string) export_settings["format"]
Though I have not really used C# that much in Godot, so there may be other ways of handling it.
