Hi,
The information at this link is explainable. The thread owner there reports that he gets redirected to
https://subdomain.local/dashboard/. The important information here is not the dashboard at the end, but the https. HTTPS uses port 443, not 80 so it will not match any <VirtualHost *:80> definitions. Instead the configuration from \xampp\apache\conf\extra\httpd-ssl.conf will takes effect, so htdocs is the DocumentRoot for this request, the index.php file in htdocs folder will get served and fire the redirect to /dashboard.
The question is, why https instead of http is used. There are multiple posibilities. Some https transformations will be done explicitly by the browser (for example Google Chrome will always transform HTTP to HTTPS / forces HSTS for all .dev domains, even if you explicitly use http:// in your url). Or maybe there is a .htaccess file or configuration of the third party webapplication that forces to use HTTPS. Or the user itself has used https:// as protocol in the url. XAMPP itself does not transform HTTP to HTTPS.