You may be correct that the two forts were omitted intentionally but I think probably not. That said, have never used the Gallipoli invasion option in the Grand Campaigns and in the 1915 scenario, both Sedd ul Bahr and Kum Kale are already occupied by elements of the British Fifth Army.
In the Grand Campaign's, both forts have entries in the Fortress.csv files and do not appear in the game because their defining entries contain punctuation errors. This is similar to the causes for the omission of the Brest Litvosk and a couple of other Russian forts in the GC. Therefore it would appear that the absence of the Sedd ul Bahr and Kum Kale forts is unintentional, otherwise why are they defined in the Fortress.csv file at all?
Anyway I am in the process of preparing updated iterations of my Naval Mods for the Grand Campaign and the 1915, 1916 and 1918 scenarios including fixes for many (all those discovered that I can fix) errors of omission and commission including adding new units, armies and leaders, for uploading here. Hopefully there may be some interest.