It's obvious that a == 0 ? false : true is same as a != 0

Also a != 0 looks more clear as a Return Type to a bool function.
It's more easier to understand what it does in this form.
WCharacter.h was full of functions which utilised a == 0 ? false : true instead of the more easy to read a != 0 which strikes a fine balance between simplicity and readability.

If we are to look at Standard Library Definitions of the various functions used like isdigit, isalpha etc. we find they return 0 when they do not find anything thus making isdigit(ch) != 0 even more clearer

Added extern C
Given the fact that we are using C Functions and code that is compatible with the C Standard, I added extern C. Extern C is also used in similar C Standard Compatible Arduino header files