First off, something like this should really be brought directly to the manufacturer to deal with rather than posted on this forum in this fashion, send an email or PM to Rob Orlando and he will deal with it. This is the second time I have ever heard of it associated with an XM-18, it can happen but it is rare.
My personal opinion, you could use the h*ll out of this knife to no ill effect, I honestly don't think the small portion of metal between the gimping and thumbstud hole hampers much of the strength of this knife but clearly the strength is less than if there was no crack. But, if it was my knife, I would have contacted Rob first to determine my options and then go from there.