The summary is that yes, this is the snap sandbox preventing arbitrary system access. The home interface provides access to files in $HOME, but it doesn’t grant open access to dotted files and directories because those often contain very sensitive information that we cannot hand off to arbitrary applications.
Per @elopio’s note above, we’ll probably offer a more general solution to open up specified paths to specific snaps in the short to medium term. This was just made much easier now that we’ve just finished the dynamic manipulation of paths inside the sandbox, so perhaps we can address it soon.