Um.
Helmets are tested with a head form that's within the range of sizes that the helmet is spec'ed to fit.
Even if the adjuster can be turned to accommodate a head outside that range, it is NOT approved or tested for those heads. Now, rules for ECE approval might be different, but I don't think so, not from what I know about motorcycle helmets.
Assuming this is a genuine email, it raises a lot of red flags to me, that the owner of the company would make such a claim.
... I still want one of those backpacks, though.
Speed comes from what you put behind you. - Judi Ketteler