Android uses a very complex scheme of virtual mount points that, in combination with SELinux, restrict access to partitions beyond what one would expect from rights management on the filesystem level.
I honestly do not understand them but I have run into those security systems multiple times in the context of running my GNU/Linux chroot aside of LineageOS. I can well imagine that also in the case under discussion here, rooting is the only way for Android Apps to access file systems touched by both OS's.