Earlier quoted context omitted.
Not to mention hydrogen fires being invisible to unaided eye in daylight.
Hydrogen at atmospheric pressure is hard to ignite in uncontrolled circumstances. It wants a specific gas/air ratio.
Might be space flight specific (ideal ratios/big flows of hydrogen maybe ?) but NASA seems to take hydfrogen fires very seriously:
https://spinoff.nasa.gov/spinoff1997/ps1.html