From what I understand, the T fitting is designed so you can attach a hose and pour the antifreeze in while filling the ballast.
I have an '08 X14. When I tried to winterize by filling through the ballast vent it didn't work. I poured in over 4 gallons of antifreeze and it never came out through the hull. I gave up and disconnected the hose from the through hull fitting and poured the antifreeze in while filling the tank. I reconnected and hit empty, and it then started pumping. I don't know if the pump loses it's prime or what was going on.....
'97 Prostar 190 - LT-1
Prior boats - 2009 X14, 2008 X14, 2005 197, 1988 Tristar 190, 1989 Prostar.