I think normally they try to send you some sort of notification that they’ve found a vulnerability, but given what I saw on the Patchstack site, that notification would likely appear to be spam if you didn’t know what it was.
They want you to sign up for their service as a plugin developer/owner and claim ownership of the plugin. Then they’ll provide the details of the vulnerability and, once you’ve fixed it, verify the vulnerability is gone and mark it as fixed.
They pay people to find vulnerabilities. They verify the vulnerabilities and then publish them. And the Solid Security plugin (which I also use) subscribes to their service.
Sounds like it’s an actual vulnerability though.
Good luck!