I would have pointed you to their issue tracker, but all I can find is a horrible Confluence wiki. Kind of discouraging for potential contributors.
There is a mailing list though. At the very least they might add a 'why not (also) JMAP' bullet point to the website if you ask about it:
> […] join the COI developer mailing list by sending a mail to dev-join@coi-dev.org.
I would guess that they need IMAP to prevent the whole 'nobody uses it' problem.