However, what are the actual chances of anyone stumbling on an eddb.io system entry with ELWs, fly out there and claim them ?
Is that something that's know to have happened ?
I think I can help you there. Out of seven thousand ELW-s, I think we had maybe a dozen cases where somebody said that they discovered a body first, but it was tagged by the time they got back. Most of those were either close enough to Jaques, or to the far end of the galaxy. Maybe one or two cases where they were elsewhere. And even in most of these cases, we can't tell whether the Commander in question really was the first to visit the system, or perhaps somebody was already there and on their way back to UC by the time the player found the ELW.
Now, EDDN doesn't store who discovered an ELW first, so AFAIK there's no way of telling from it whether a given ELW is untagged yet. Of course, we have no idea whether there's anyone actively looking out for ELW-s that appear there to go claim them, but the farther out said planets are, I'd say the less likely they'd go out and check to see if they can be claimed. To be honest, in the time that would take, they could probably find some of their own. However, EDSM
does store who visited and submitted the system first, so there is that.
To sum it up, I'd say the chances of them getting nabbed are very low, but if you are concerned, it's better to be safe than to be sorry.