How to test if my files are not accessible from the internet?

That warning is generated here:

and is thrown if the environmental value htaccessWorking is not set.

That vallue is set in the .htaccess file in this line:

if mod_env is loaded in apache2.

Since you are on a hoster, it might be possible that mod_env is not loaded and that would explain why you got this message. Nothing else!

If you have got full access to your webserver, you could find out if mod_env is loaded with this call

~$ sudo apachectl -M 2>/dev/null | grep env_module

it should echo this:

 env_module (shared)

Much and good luck,