I think the project that ended up spurring ESF into motion was called "Power Up" (EDIT - Looks like Dam beat me to it
)
I don't remember the exact association; I don't think it was as simple as that the ESF team (at the time) was working on Power Up and decided to shift gears with the project. I think Masta and a couple others were associated with Power Up, and decided to go off and do their own project when Power Up wasn't going anywhere.
At the time (2000 - 2001, I think), HL would have been one of the most (if not THE most) prevalent platform to mod from, so it's no surprise that ESF ended up as an HL mod.