Here is the hard truth: It is written in C# and relies on the .NET Framework, which is native to the Microsoft Windows ecosystem.
Valve has historically taken a "hands-off" approach to achievements. Since achievements do not affect the gameplay or competitive balance of multiplayer matches, modifying them is not treated as a bannable offense in the vast majority of cases. However, use SAM at your own risk. Valve can change their policies, and while you won't get a VAC ban, you might find your achievement stats scrutinized more closely by the community (e.g., sites like AStats). steam achievement manager mac
Ensure the game you are modifying is completely closed before using any manager. Here is the hard truth: It is written