I hope they have anti-sloshing-baffles in their design, because liquid hydrogen has low viscosity, and you really do not want it sloshing around without energy dissipation, in particular not during take-of or landing.
But a low-speed piston seal that only has notable pressure across it during active roll maneuvers (centripetal force from the roll-rotation) and uncoordinated turns, between LH2 and gaseous H2 with the latter slightly warmer than the former... it's not like you'd want the empty part of the cryogenic tank to be at non-cryogenic temperatures or filled with something that'd contaminate your LH2.