That is, what made it Smalltalk's git, and not, say, Smalltalk's CVS or ClearCase? The linked-to page suggests that ENVY used a centralized system. (Eg, "The Git model is better for distributed development" and "it had a small process that ran on a server to manage record locking".)
Consider also some of the limitations in ENVY. "You would bring your changes to the machine in the form of (IIRC) a list of RCS-like versions of individual methods, then review and merge them with latest." and "What wasn’t available, was automatic merging".