Again, what does your Browser Adress bar say?
A index.html file in your root htdocs folder SHOULD NOT be affected by the new security concept.
I believe, that you don't request the index.html file directly by the full url (including a index.html at the end). And if you don't do so, not the index.html file is loaded, but the index.php file which directly redirects you to the htdocs\xampp folder. Buit without giving us the information we need, only you have all the information needed to solve your issue.
P.S.: Do you try to access your file with your computername in the url?