So the high Isp is due to the very light exhaust (hydrogen, not water or CO₂) which gives the higher velocity to the gas molecules at a lower temperature?
In a sense, yes. For a given temperature, hydrogen, with 1 atomic mass unit, will have a much higher velocity than water (18 amu, so √18 ≈ 4.2 times as slow) or carbon dioxide (44 amu, so √44 ≈ 6.6 times as slow).
Yes, having lighter exhaust also helps you raise your ISP. Scott Manley has a few videos on this topic, where he explains the science behind this much better than I ever could.
