I think GP is referring to the fact that email overall is a system that is based on public standards and open to new entrants. You can start Hmail.com if you want, and plug into the existing email eco-system as a new competitor very easily.

The social media ecosystems aren't like that. You can't be a chat provider and plug into FB Messenger; you can't plug into Twitter, etc.

There is an open social media eco-system called the fediverse (for its federated nature), in which Mastodon is the best-known player. But it's gotten very limited traction, because of the network effect that keeps people on FB and Twitter. No such effect keeps people on Gmail.

Email, not Gmail. I can email people from my provider even if they use other providers, including people who self host. And I can get email from them too.