I don't think the author meant "WTF" to be a constant. It was defined to be whatever complexity index creation has. However, I still disagree with "O(WTF*N)",

I don't think the author meant "WTF" to be a constant. It was defined to be whatever complexity index creation has. However, I still disagree with "O(WTF*N)", because "WTF" almost certainly depends of the number of rows in the table (large tables take longer to index than small tables), and that number becomes lower in the multi-tenancy approach, because the individual tables are smaller.