Because his client is claiming absolute immunity. So he has to hold that position even to the utmost absurd hypotheticals. If there is anything at all that isn’t covered then the immunity is not absolute, and he’s forced to argue degrees about which acts or crimes are covered. At that point the entire argument collapses as the result is quite obviously “at least some acts” and “at least many if not most crimes” are not covered by any sort of immunity. There’s nowhere else to run with the argument.