I think no13 must be intended to give a launching point to go through Loders Cut. I feel like it’s been there or not several times over the years. I also feel like it’s been a red before.
(I once anchored on the mud flats there directly in front of the drying island just for a couple of hours either side of HW with 1.5m draft and felt very smug to have found an empty spot all my own. Luckily, after lunch I got away before Neptune spotted my loitering.)