(Dor posted elsewhere that the failure of this lamp was sudden, after the lamp worked normally in previous times. That is relevant for the question here)
I think it is unlikely :
If the lamp was ok initially it means that it had no Hydrogen trapped in it from manufacture, and no small leak that would let Hydrogen (the smallest atom of them all) to penetrate over the (typically) few months or longer time, when the lamp was in storage, before you got it
The water gets released from the quartz slowly, so it slowly poisoned the arctube. This caused the cold cathode glow voltage to rise slowly. Up to some limit the mains (= the ballast OCV) was still large enough to pass sufficient current to warmup the electrodes (it is a kind of unstable system - small heat causes electrodes to slightly warm up, that lowers the voltage a bit boosting the dissipated power, that warms up the electrodes even faster, so the system flips into the hot electrode mode when the electrodes are fully warmed up so have low voltage drop), so the lamp started apparently normally. Only when it just exceeded the critical concentration, the dissipated power in glow mode was insufficient to make the system to flip into the hot cathode mode, so "from the outside" it "suddenly" stopped working.
I dont think any gas can get *into* a HID lamp arctube, even when it is leaking, when the lamp is on, as the pressure inside is higher than outside. If anything, something would be leaking out...
Even when external gas leak isn't likely the cause here, you are wrong.
The gas will leak in (assume very small leak, more of a diffusion nature than real hissing), when the partial presure of that given gas is lower within the arctube than in the outside. The pressure of the other components does not matter there. So because there is supposed to be no hydrogen inside at all, even a small pressure outside is way greater than the H2 pressure inside of the artube, so the hydrogen will diffuse into it even when the other gasses have there 10atm... Of course, this is valid only for small, diffusion like leakages, when all flows are subsonic. If you have really a hole causing hiss, the hiss is a supersonic flow and there the supersonic flow really stop anything else going in the opposite direction.
In real life I've not heard about H2 going this way, but He poisoning is a real problem, becase He is a single atom gas with very ligt atom, so really diffuses through most practical construction materials. And there a small percentage of He in atmosphere (some He is leaking or even just in use in that room) has no problem contaminating a presurized container with another gas stored in that room. It takes time, but it does happen.
The same mechanism is actually able to save your day when He contaminates some MEMS structure (they are supposed to operate within vacuum, the leaked in He causes excessive damping so the MEMS fails; typically 32768Hz RTC oscillators used in modern smart watches or smart phones). When you move the affected thing outside to a clean air (not contaminated with the He), the He leakes back out from the mems component to the outside air (because the partial pressure outside will be lower) and the thing recovers and starts to operate again normally (unless the software collapses and cause havoc in the system over the time when the RTC did not work), even when the N and O make together 1atm difference in the total pressure.
