It was, but not until over two years later as a Mac port of a Windows port of an Xbox title. That was done by MacSoft, Bungie (presumably) having been tasked with sequels on Xbox as soon as resources freed up from the original.

After the Bungie sale, Xbox was the primary platform they designed for. Before that it had been planned as a Mac/PC title.

I still hear the rocket launcher occasionally.

Somewhere I have DATs with like 1GB of custom Marathon levels and texture designs. Including a complete model of our Junior High School a friend of mine made. I'm not sure how I missed the announcement they open-sourced them.