Spammers likely don't care 'who' the person is on the other end. If someone receives their spam, then they succeeded and they know that address works.
You missed my point about addressee responses and server responses.
During the transaction when the sender (legit or spammer) connects to Groups.io to deliver the message Groups.io has only two choices: accept the message or reject it. Marc's evidence is that Groups.io is accepting the spam message (there was no failure notice at the sender). Either way though, the spammer learned that Groups.io's server is functioning - which is unavoidable.
If Marc's evidence is correct then the spammer now has every reason to believe that the message was delivered to the addressee. A deferred Bounce message from Groups.io might cast doubt on that inference; but I agree with your assessment that typical bulk spam sources probably won't process such responses in any meaningful way.