How can I be pwned if I'm not registered on the compromised site?

From the FAQ:

Why do I see my email address as breached on a service I never signed up to?

When you search for an email address, you may see that address appear against breaches of sites you don't recall ever signing up to. There are many possible reasons for this including your data having been acquired by another service, the service rebranding itself as something else or someone else signing you up. For a more comprehensive overview, see Why am I in a data breach for a site I never signed up to?

It's likely some services allow signing up without confirming an email address, or that accounts that haven't confirmed email addresses are still stored indefinitely but cannot be logged in to, or any number of similar issues.

Adding on to what AndrolGenhald said, they have deactivated all accounts associated with the breach so theres a good chance it won't show up regardless:

ShareThis has already deactivated the ShareThis accounts potentially associated with this incident, so if you created an account prior to January 2017, you may no longer be able to log in.