Need not be. The important thing with mutt is that it's standards compliant, it helps follow the conventions of these communities, and as a plus it's easily configurable and scriptable. If there are GUI tools that bear the same properties, they are equally viable (AFAIK Claws comes close).
"Many people, especially younger devs, have never seen a real mail client like mutt. In their minds they may picture an overflowing GMail inbox full of top posted replies all mixed together from different projects. That’s not an enticing image. Perhaps this article can start these developers on the path to rediscovering the care and engineering that went into classic email clients."
My point is that this is nostalgia for terminal-based email clients.