I haven’t tried setting up IMAP directly in mutt for a decade, so maybe Neomutt has made improvements, but it was often laggy to the point of being unusable.
I’d suggest people look into using mbsync (author mentions it at the bottom of the blog post as an update) as an external tool to sync mail via IMAP, or if you’re really committed to Gmail you can give lieer a try. It integrates well with Gmail’s labels and syncs them with a notmuch database. I stopped using Gmail years ago and switched to Fastmail. There’s an equivalent program to lieer called mujmap that works with Fastmail. I’ve been using it for over a year and works great.
OK, so I haven't used Neomutt (or mutt) in ages, but I assume it's about as capable as aerc, which I do use. Aerc is an alternative to Gmail in a couple of ways:
If you interpret the meaning the way you did, aerc can access a gmail account via IMAP, just like any IMAP client can. So it's a replacement for GMail web
It can be configured to use notmuch, which is the most GMail-like interface to mail in that it indexes email for searching and uses tags instead of folders - like GMail. So it's a replacement for GMail in that it behaves the most like GMail without being backed by GMail (although, you can connect it to a GMail account email corpus if you like).
It can talk to any IMAP server, so it's a replacement for GMail in the purest sense: there is nothing GMail involved.
I suspect Neomutt is equally capable; I've just always disliked the UI.
What were you expecting? A promo for ProtonMail, or a replacement as in, "hey you can go here and register an email account, so it's a replacement for GMail?"