Hmm.

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:

1. 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
2. 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).
3. 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?”