My limited understanding is the location/country of the proposed violation or the violators need to be a signatory. In this case, Palestine is a signatory so the actions of Israel in Palestine as well as the actions of Hamas (acting anywhere) are within the court's jurisdiction.
The "State of Palestine" (the official name in this context) is a signatory to the ICC, and is the location of the committed crimes. I guess you could technically make an argument that despite Sderot et al being an occupation under international law, because they are in the actual control of Israel which isn't a signatory then crimes committed there couldn't be prosecuted by the ICC. Given that the only party who would try to escape arrest for crimes in those areas are Hamas leadership, I doubt that's what Israel wants. Also, I doubt Hamas is willing to legally state any part of Palestine is "Israel", particularly for a relatively non-existential threat like an arrest warrant.