Hello Wim,
the solution you found is the correct one.
Regarding security, I can only say that in this way you won't be protected by Admin Tools. If there's a security issue in such file, Admin Tools won't be there to mitigate any threats.
You should contact the developer of such extension and tell him to follow Joomla guidelines, since every request should be addressed vs Joomla main file, the index.php one.
Davide Tampellini
Developer and Support Staff
🇮🇹Italian: native 🇬🇧English: good • 🕐 My time zone is Europe / Rome (UTC +1)
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!