Honda Global | Honda R&D Co., Ltd., a research and development subsidiary of Honda Motor Co., Ltd., today conducted a launch and landing test of an experimental reusable rocket*1 (6.3 m in length, 85 cm in diameter, 900 kg dry weight/1,312 kg wet weight) developed independently by Honda. The test was completed successfully, the first time Honda landed a rocket after reaching an altitude of nearly 300 meters.
I suspect it’s because the hydrogen molecules are bigger than a single helium atom, which doesn’t form molecules (since it’s a noble gas).
So the hydrogen molecule only seeps through if it’s oriented right (since the hydrogen molecule is a stick-shaped molecule).