Complex sentence: To this they reply that God is omnipotent, even though there are states of affairs he cannot bring about: omnipotence is a matter of "maximal power", not an ability to bring about all possible states of affairs.
Split into simpler sentences: To this they reply that God is omnipotent even though there are states of affairs he cannot bring about. For, on their account, omnipotence is a matter of "maximal power", not an ability to bring about all possible states of affairs.