API to fetch all the function excel supports

You could use PowerQuery to get the list from this page: https://support.microsoft.com/en-us/office/excel-functions-alphabetical-b3944572-255d-4efb-bb96-c6d90033e188 

I don't think there isn’t an API that returns a full list of Excel functions. In the Office JavaScript API you can use workbook.functions to call them, but you’d still need to know the function names up front. The built-in Function Library in Excel is the closest thing to a complete list.