Don't use pamac then. Just use pacman or any AUR helper with pacman command wrapper like yay or trizen until Manjaro team include that repository to their mirror list. I think pamac will overwrite
/etc/pacman.d/mirrorlist if you're doing a mirror refresh.
Regarding pamac, in my opinion, pamac (especially the GUI) are too obstructive, slow, and annoying to use. I don't get why it always trying to update and sometimes re-download the package database every time I access the Updates tab. It's counterproductive and also wasting time and bandwidth. Pamac needs a lot of UX improvement to be honest.