>>>The 20 million short haven't been used yet right?
That is not clear to me. The short interest is the number of shares that were cumulatively short and the transactions cleared by those mid-month and end-of-month dates. Since the cumulative number of shares short in the market went down, that means shorts bought to cover the shorts and returned the loaned shares.
The days-to-cover is a measure of the existing short position. Not an available short position, but an existing short position. 20 million shares have been borrowed and sold. They must be purchased and returned. If covering shorts buy 4 million shares per day, it takes 5 days to cover. It is just a means of relating the short size to normal volume to determine if the short position is large. It is large relative to the float, but less large relative to volume.