Q: why an ingnition timing at idle is 5 BTDC at base haltech maps?
 

I suppose it must be 5 ATDC for leading and no timing split.
Am I right?

5 degrees ATDC is whats on the stock pully for the leading timing, what mazda recommends at idle, and 20 degrees for trailing. Which means, mazda recommends a split of 15 degrees. But, depending on who you talk to and what you want to accomplish, the split difference will vary throughout the entire rev range.