I suspect Pine ships Manjaro ARM because that distro will apply nearly any patch to the kernel if it makes something work, no matter how hacky or unsustainable the patch is. Right now their kernel has 43 patches, and they are not even organized or annotated well.
They ship Manjaro ARM because the people behind PINE64 are manjaro fans and they think that Manjaro has a good business setup. It would have been much nicer to have PostmarketOS shipped stock.
https://gitlab.manjaro.org/manjaro-arm/packages/core/linux