Even if some models are able to find some large fraction of the vulns that Mythos finds, the contention (that those of us outside Anthropic and a few select partners do not yet have the ability to replicate) is that Mythos not only found vulns, but was able to string them together into working exploits, very close to autonomously or at list with minimal hand-holding. I believe that UK AISI has at least in part confirmed that Mythos is better at this than other models like GPT 5.5, even while it's not better at the simply finding vulns part (although, again, no one else is currently able to replicate their results).

This was the primary reason for not releasing it. The difference in the two primary camps around this topic are that the doubter group thinks that Anthropic, and all of their partners, are essentially lying about this (since no one outside Anthropic and select partners has the access to replicate), whereas the other side believes that Anthropic and partners are probably mostly telling the truth without too much exaggeration.

Neither camp has evidence other than unconfirmable reports and/or arguments about economic incentives. I personally think that Anthropic has, in the past, mostly not lied about things like this and has by far been the most transparent and open AI company. That could change, and they could be lying now, but I think that the camp that is certain that they are is far, far too confident in their belief.