It seems more than a tad rude for them to try to circumvent IETF
in an area (email standards) that is traditionally IETF territory
Email best practices is not "traditionally IETF territory".
Email protocols are, and surely the largest concentration of experts
in IETF email protocols is here. Also, when I've seen other efforts
outside IETF to try to write documents that specify how to use IETF
email standards, they inevitably botch some thing or another.
That's why I think this work should be vetted in IETF. And the time
to do that is not when the originating group thinks their work is
finished (and cast in quick-hardening concrete) but when they think
they have a general outline of what is desirable and it needs wider
review. Which, from my reading of their work, seems to be about now.