For this phrase see already under alphaiota 23.
At first sight it makes no sense, because the Aegean Sea contains numerous gulfs, but attested usages -- all post-classical -- are of the pars pro toto kind. (See e.g. Philostratus, Imagines 2.13.1, describing the Gyrai rocks as being tou= *Ai)gai/oun ko/lpou; Hesychius gamma1024 says, more precisely, that they are in the Ikarian Sea near Mykonos.)