The author uses MSYS2 which is based on Cygwin: https://www.google.com/search?q=msys2+based+on+cygwin

EDIT add : >I wonder why that is not an option anymore.

In the particular context of the blog, the author deliberately wants to make MSYS2 the main environment when he wrote: "These prerequisites are not just to use mutt, but to use msys2 successfully in any capacity for development or other work."

Therefore, installing Cygwin separately for Mutt would work but it contradicts the goal of him wanting to use MSYS2.

The author probably prefers the MSYS2 way, but Mutt should work fine under Cygwin without requiring jumping through the hoops the author describes.

Yes, this fragment is what my reply was about.

>it is not Cygwin (as opposed to how, say, Ubuntu is based on Debian).

I agree and that was not what my reply was claiming.

Instead, I interpreted the gp's comment as being unfamiliar with MSYS2 and how it's related to Cygwin to fill the use case for a UNIX-POSIX-emulator.

Extra trivia... as for whether "MSYS _is_ or _is not_ Cygwin" is a matter of context and particular person's perspective for comparison. Examples:

"MSYS is not Cygwin": https://news.ycombinator.com/item?id=11390890

"MSYS is Cygwin": https://news.ycombinator.com/item?id=11390961