The split feature you are trying to use is designed for strings, not Postgres arrays.
You could have postgres convert the array to a string and then split that, but you will only be able to split a fixed number of columns. That probably doesn't work if you have a mixture of column widths.
More likely you want to convert the postgres array into a set of rows. This would require some custom SQL using postgres's unnest function.
Hi Dan, thank you for your suggestions.
I ended up solving it by using a custom SQL, solving this inside Tableau seemed too difficult for me.