It does use helium only as refrigeration fluid in closed cycle, it does not get depleted. Building a cryocooler capable of achieving so low temperatures without boiling helium off was a major hurdle in JWST development.
Isn’t helium notoriously hard to contain without leakage even in a closed system?
that's hydrogen. IIRC from my chemistry days, Helium is hard to contain in plastic closed systems, but "generally OK" in metal tubing.