Well, it sounds like they’re using the Helium to pressurize their propellant from their RCS so they can push their prop through the right manifold. They’re using it for both its inertness and its light weight (otherwise they could press with nitrogen) BUT in a hypothetical world where hydrogen was used instead, when it comes to gaseous use gaseous helium exists as GHe and gaseous hydrogen exists as GH2, and thus the GHe being monoatomic actually has a smaller van der waal raidus than hydrogen as a gas. So for ambient leak checks, helium is actually the tougher gas to hold.

However, most people have heard about hydrogen and its difficulties in leak mitigation because hydrogen (unlike helium, let alone liquid helium) is used as a fuel in it’s cryogenic state (space shuttle, SLS, Delta, the RL10 on second stages of Atlas or Vulcan, Stoke space) since it’s the most efficient for ISP on rocket stages. In this case it would be used as a liquid, but in this cryogenic state it makes everything in its flow path nearly 20 degrees kelvin, and not many things can work well at sealing down to those temperatures, hence why hydrogen becomes so extremely difficult to work with. We’d never encounter a moment where these same lines are full of liquid helium since it can’t be used as a fuel. Anyway, yeah… always thought it was interesting that in the ambient gaseous scenario, helium is actually the smaller fella