Yeah, but the target of Proton’s efforts with Gluon is the clients, since Gluon is part of the ProtonMail Bridge software.
Their primary targets are Microsoft Outlook, Apple Mail and Thunderbird, none of which speak JMAP at present. So Proton has very little to gain from implementing JMAP, since only niche clients support it at this point.
Classic chicken and egg problem, unfortunately.
Worth mentioning that Fastmail use an open source backend: Cyrus[0] - which started adding JMAP support in 2015 (though I suspect that was Fastmail devs contributing the feature).
[0] http://www.cyrusimap.org/