The article covers the fact that while you can in a literal sense setup your own mail server, the mechanisms used internet wide to curb spam (the reputation systems) will penalize you for doing so as compared to using an ESP like Mailchimp.

I read the article. It depends on your use case but you most certainly can successfully run your own small mail server with your own domain. I've done it for many years without issue.

It's the other side of the same 'openness' coin. You can chose to set up your own mail server. You can chose not to deliver email from other mail servers if you want.