Hi @sixer,
The plugin checks if the class Memcache exists in the system. If it does, then it does the Memcache as enabled else it says it is not. It simply runs this basic check class_exists('Memcache') to check for the Memcache class: https://www.php.net/manual/en/class.memcache.php

You can dis deeper into your server to check why class_exists('Memcache') is returning false despite it being enabled. Better to ask your host about it.