Not that tightly linked. Whilst the Element client does offer Matrix.org as a signup/login as the first page you see, you're also just one single click from signup/login to any of the other servers. Nothing about that page really feels locked or branded to matrix.org.
I don't see how that makes Element responsible for matrix.org content. Thunderbird offers getting an email address via gandi.net in their new account page. Does that mean that Mozilla is suddenly responsible for all emails (like spam) coming out of gandi?