They are, but DNS is not used to redirect slow.com to fast.com. They both resolve to different IP addresses. It's only when the browser requests "http://slow.com" from the slow.com server that it is told to redirect to https://fast.com/. This is an HTTP redirect, and has nothing to do with DNS.
The context was "you may change it", where "it" is the redirect from slow.com to something other than fast.com. But he wouldn't be changing any DNS entry as he's still resolving the hostname to the same IP address; instead he changes the http redirect to some other site. No DNS entries need to be modified as part of "you may change it".
I'm not sure why, on a site called Hacker News, that you're surprised on being called out for a technically imprecise statement. Instead of digging in your heels with bogus justifications that you're rightfully being downvoted into oblivion over, why not thank those who replied and offered a more technically accurate explanation?