If you get too big, you can't buy your competition (e.g. FB buying IG). Or if you get too big, you have to open your stuff up like email does. Or if you lie to congress, you get penalized. Or if you get too big, you have to make your algorithms publicly available.
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.
