> Do you think a human is capable of providing assistance with defense but not offense, over a textual communication channel with another human? > If no, how does a cybersec firm train its employees?

In general, no, humans can’t be sure they are only helping with defensive and not offensive work unless they have more context. IRL, a security engineer would know who they’re working for. If they’re advising Apple, then they’d feel pretty confident that Apple is not turning around and hacking people.