That post was made before the game even launched when servers were planned to have upwards of five layers. They updated two days after that post saying this:
Do you believe this now? Things change, those posts are long outdated and referenced characters created before Classic even launched. Characters created =/= people playing. Here’s Sul’thraze Alliance at about 6:00 in the morning a week ago:
https://i.imgur.com/QHu0h9s.jpg
No server has 12k+ players logged online at any time right now. You can easily check this for yourself with:
https://www.curseforge.com/wow/addons/censusplusclassic
Servers are capped at 5,000-6,000 players per layer. No server is “Full” right now, which means no server is hitting that cap.
I’m arguing because misinformation is rampant on these forums.