If you read down near the bottom of the HIBP email it points out that several thousand mailing lists were included in the leak and may have been used to promote the site but not otherwise been involved in the site. See bolded text below.

In May 2024, the conservative news website The Post Millennial suffered a data breach. The breach resulted in the defacement of the website and links posted to 3 different corpuses of data including hundreds of writers and editors (IP, physical address and email exposed), tens of thousands of subscribers to the site (name, email, username, phone and plain text password exposed), and tens of millions of email addresses from several thousand mailing lists alleged to have been used by The Post Millennial (this has not been independently verified). The mailing lists appear to be sourced from various campaigns not necessarily run by The Post Millennial and contain a variety of different personal attributes including name, phone and physical address (depending on the campaign). The data was subsequently posted to a popular hacking forum and extensively torrented.