That reddit thread is horrible advice, it’s just mapping the LXC root user to the host root user, which is just an privileged LXC with extra steps.

The reason you’re probably having issues is that your root user in the LXC is mapped to the host user 100000 by default, and that user doesn’t have access to the share, but you can change that with mount options or creating a user with 100000:100000 and adding it to a group with access.