“Hydrogen gas leaks through everything and can accumulate to dangerous levels if batteries are in a confined space without ventilation.”
If it leaks through everything it can’t accumulate!
Differential rates of leakage from a small container into a large compartment will create temporary accumulations in the larger compartment. The larger compartment at lower pressure will leak at a slower rate.
If you hit the correct ratio of free hydrogen to free oxygen in the larger compartment, you are ready for an explosive detonation at the first glimmer of static electricity. Or the turn of a light switch.
That is why rooms containing high-power lithium batteries must be ventilated.