"Hand" here is a reference to military strength. AT: "As my powerful army has defeated" or "As I have conquered" (See: [[rc://en/ta/man/translate/figs-metonymy]])
During this time people believed that the greatness of an idol showed how powerful the kingdom that had built it was. The king of Assyria is saying that because the idols in Jerusalem are not as great as the idols of the kingdoms he has defeated, Jerusalem would be less able to defeat him than they were.