A “fork” is when someone makes a copy of the code, edits it, and the results are not incorporated into the main project.
In this case, grbl was copied and edited to work with the Rambo, but the changes were not brought back to the main grbl version. The consequence is that the software has a lot less testing (grbl has hundreds of thousands of users, the fork has significantly less, maybe a few hundred). Any updates or bug fixes on the main grbl may not be fixed in the fork. Bugs introduced in the edits on the fork may not be found.
Support in the main branch for many many boards (including the Rambo and skr pro) are a big benefit of Marlin.
I have not tried it, but I think there is reasonable support for exporting gcode from easel and use it with Marlin.
That said, there is a long post here where people have gotten the fork to work and others haven’t:
The code is called “grbl-mega-5x”. I am not sure which fork is the best, or if anyone is trying to keep it up to date.