Yes, I believe that was the intended usage:
$ git clone git://git.yoctoproject.org/meta-lts-mixins -b dunfell/kernel meta-lts-kernel-mixin
$ git clone git://git.yoctoproject.org/meta-lts-mixins -b dunfell/gizmo meta-lts-gizmo-mixin

When TSC discussed the process for Mixin layers, we wanted them to be slim
single-purpose layers and using branches instead of whole git repos seemed
reasonable. Especially since we were not envisioning too many of them. But
if it proves to be a problem in the future, we can certainly revisit it then.