Delta does not offer free domestic Economy Comfort at T-24 to it's Silver Medallions. You are confusing it with United's policy of offering Economy Plus for free to it's Silver's at T-24.
50% OFF for Silver Medallion or free for Silver Medallion within the check-in window
http://www.delta.com/traveling_check...my_comfort.jsp
AS is not eligible for free domestic or short haul international, only TATL and TPAC.