But I do suspect that at some level the hydrogen loses its electron when inside the metal, and becomes a naked proton, only regaining it when it re-emerges. And a naked proton could actually go deep into metals normally considered not to adsorb/absorb H but without affecting their gross physical structure in a readily detectable way.
'Ye olde theories' did indeed talk about bare protons swimming around within the electron clouds of the bulk metal lattice. There may still be some portion of truth in that model - as well as in all the others (migration to grain boundaries, H2 recombination, etc).
For years I had been relying on a fairly basic rule of thumb that low carbon steels were hardly affected, but had become fairly paranoid over the behaviour of high tensile alloy steels, when exposed to any form of hydrogen (gaseous, or aqueous) - after experiencing some nasty (and potentially catastrophic) component failures.
Interestingly, the webinar presenter was claiming that practically any metal or alloy could be affected. This was despite the fact that there has been a high pressure hydrogen pipeline in operation, linking various German chemical plants, since 1939 - and is apparently still functioning well (someone in the live comments claimed that it was made out of mild steel).