The parent is talking about communicating with friends and family. My point is not about evading surveillance, it's about not using one of the main tools of the CCP censorship apparatus.
If you're inside China and care about your safety, you would think twice about sending "questionable" messages even on the most secure channel because you're still physically vulnerable to rubber hose cryptanalysis.
If anybody messages you or your family/friends receives something suspicious something and the Party suspects you're intentionally avoiding their watchful eye with a custom mail server, I think you might end up worse off. All it takes is one goofball finding a vulnerability in your system and dropping a joke image or bit of text to royally screw you.
Furthermore, while it's nice in theory to deploy your own servers and get all of your friends and family to exclusively chat through your homemade application, it's very unlikely. WeChat is basically an OS all its own these days. It's a social network, chat app, payment app, shop, and more. People are incredibly reluctant to give up convenience unless they're very motivated and technologically inclined. And for people inside of China, getting a VPN or non-Chinese messenger is quite difficult thanks to locked down app stores and most people communicating only through phones.
I don't use Facebook and won't budge on that issue. My parents won't use anything that's not Facebook. If they won't take 10 seconds to register for anything else, then they certainly won't want to deal with anything I'd try to scrap together. The end result is that I make a VOIP call to their phone about once a month and they ask me to just give up and use Facebook at some point. It's probably a similar situation with most people wanting to leave XYZ terrible messenger.
If you're inside China and care about your safety, you would think twice about sending "questionable" messages even on the most secure channel because you're still physically vulnerable to rubber hose cryptanalysis.