Assuming you run a legit SSH server on port A and an SSH tarpit on port B. What hinders an attacker to connect to port A an B at the same time? What is the advantage having an attacker connected for weeks on port B?
They will usually try port 22 for SSH, only. So, if tarpit is on port 22 and real SSH is on another - mission accomplished: you're making them (at least one of their threads) wait.