A lot of this type-system stuff is not really relevant to the Fortnite modder audience. If you look at the kind of code used in more practical examples (https://dev.epicgames.com/documentation/en-us/uefn/learn-cod...) it doesn't do very much of this -- it's basically Python crossed with Java.
Well, they have also inverted the usual capitalization conventions for seemingly no reason. Class names are lowercase_with_underscores and local variable names are UpperCamelCase.
I'm just guessing, but snake_case might be inspired by dependently typed programming languages, where there's no strict boundary between terms and types.
PascalCase for variables reminds me of unification-based programming languages like Prolog.
Well, they have also inverted the usual capitalization conventions for seemingly no reason. Class names are lowercase_with_underscores and local variable names are UpperCamelCase.