If so, they haven't Open Sourced the implementation they're using (they have open sourced a JMAP-IMAP proxy that is pretty nice, but I probably wouldn't want to use a proxy in production). I don't recall what they've said about how they're using it, and if they're using it or some precursor to JMAP on their production systems.
JMAP isn’t all standardised and set in concrete yet (there’s an IETF working group for it [1]), so we’re not enabling JMAP for the general public just yet, nor is our branch of the FastMail web UI which uses JMAP deployed.
I was under the impression that it was more or less running live at FastMail but I admittedly never dug into the details.