Permissions on folders vs files?

You can't access/enter a directory (or create files) with permissions set to 600 as a regular user. You are also not able to acces/list (well, sort of) files at all with said folder permissions.

If any of the files has additional hard links located outside the 600 directory, they will be able to access those files using that alternate path. But this won't give them access to any of the other files.

But if the hard links don't already exist, there's no way for them to create them, since creating a hard link to a file requires access to the directory containing the file.

Given that there are no other mechanisms (e.g. ACL, xattr?) play a role then on a extX-file system you should be save. Nevertheless, a careless modification to the permissions or the directory could leave your files exposed.