That's BS because AC privilege does not attach to ongoing or future criminal acts.
If this is an honest statement (which is of course highly unlikely), the offer of immunity grossly premature. Comey could have subpoenaed the laptop first. If the court quashed the subpoena, appeal the ruling. If he loses on appeal, he can still offer immunity then.
Prosecutorial malpractice at best. Blatant corruption more likely.