[T-A]

Sorry, saw this only now. The author is assuming that the spherical shell is a topological defect in some Higgs-like field. Generally speaking, if the coupling constant(s) of a field are large compared to gravity, it's reasonable to ignore gravity when looking for solutions to its equations of motion (just like you usually ignore gravity while doing electric engineering). Once you find one, you can switch on gravity and verify that it still holds. In the case at hand, it's all conjecture, since the author just assumes existence of a suitable solution.