Well, you suggest a workaround for the problem 
Indeed SQL databases may have various ways to retrieve the information about the actual database size on hard drive, however the issue I observe is in particular in the way how path resolver for sensor.file_paths vs homeassistant.whitelist_external_dirs works. And they work differently. So the post is more to draw attention to the bug, rather than find workarounds or alternative solutions. One straightforward solution would be to use bind mounts instead of symlinks – I am pretty sure the sensor will work fine.