uucp and the internet were not the same things - but both hosted usenet and mail.
The big issue with commercialising the internet was really that originally internet connected companies were prohibited from routing packets to 3rd parties, what really changed things was the change of that policy - after that anyone could join
Of course uucp and the internet are different, but seismo, later uunet, run by the same person, were central hubs where they connected, and ftp.uu.net was one of the main software distribution points. Before seismo became the most popular, it was Bell Labs ihnp4 (Indian Hill), decvax, ucbvax, mcvax in Europe, etc.
That's why email addresses in signatures and mailing lists were usually expressed relative to seismo like "...!seismo!foo!bar!baz!user" (pure uucp routing), or "foo!bar!baz!user@seismo" (mixed internet/usenet routing), or later "user%foo%bar%[email protected]" (uunet gateway with fully qualified domain name), because everybody knew how to get to seismo (later uunet).
only if you happened to live in that geographical area (and seismo was a close routing) - I still have business cards with a ucbvax relative address, never used seismo
The big issue with commercialising the internet was really that originally internet connected companies were prohibited from routing packets to 3rd parties, what really changed things was the change of that policy - after that anyone could join