It is Gmailify serving the "unlimited", not Google. No ToS violation - not using Google's APIs. Gmailify is an extension of existing features of Gmail.
Many Gmail users already do this, but through a complete service like Fastmail, Migadu etc. Gmailify provides a thinner layer made for this purpose specifically.
Not me being dismissive, just me reconciling the definition of "unlimited" with the very limited constraints of Google's APIs