I think LEJOS team has made their original work.
However, I guess that they had used LEGO Mindstorms NXT Hardware Developer Kit as the reference, and an ARM7 expert implemented LEGO equivalent functionality.
While I was hacking LEJOS firmware to make LEJOS OSEK, I found a funny difference between LEJOS firmware and LEGO original one in I2C communication. According to the Hardware Developer Kit, communication rate for I2C is 380kHz and LEJOS uses this, however, actually LEGO firmware uses 400kHz. So LEGO might change the rate after the release of the Kit.
My general impression on both firmware was, LEJOS firmware is developed by an embedded control software expert and LEGO firmware is more UI driven software. Anyway, both are great!