ok, so the actual Lightning devs seem to be saying that real-world instances of the exploit have happened, not just a bunch of whiners from competing cryptocurrencies. That's not to say that someone couldn't be deceiving the devs, you have to take these claims on trust, as it'd be simple to falsify any evidence. But I guess that must be the underlying point: some person who the Lightning devs genuinely trust have reported this to them.
Is there any indication of how much has been stolen according to those making the claims? Tbh, I wouldn't expect the devs to come up with such statements if they haven't been given the data to review what actually has happened. If they did anyway, then that's a very poor show from the devs because it's more fuel to those looking to leverage it against Bitcoin.