I don't know what the deal is there, but I can tell you ArchLinuxArm has worked pretty well in the experience of several people who took my advice.
ArchLinux maintains bleeding-edge everything

and calibre is no exception. ArchLinuxArm already has an
ARM build of calibre.
(I have issues with the parent project's PKGBUILD, but at least they also make it extremely easy to rebuild it yourself and fix those problems.)