ACT reads the data between the client and server to determine what your parses are. ACT writes to your harddrive and never sends its information to the FF14 servers, ACT also does not write to your game client. ACT displays its information with an overlay which is effectively another window opened on top of the client. ACT does not try to gain access to anything without your permission and only has network access when you're updating the software or uploading your parses to the website.

The Mabinogi hack you're referring to is people actually hacking the packets of information to find a weakness to exploit and then sending information to the server to delete character information. The Mabinogi hack is different in almost every conceivable way from ACT. The only similarities to them is they were both written by a third party.