I'll go even simpler than the others: you're being given completely subsidized early access to the latest and greatest model. Why not take it?
I'm not saying they're lying about it being a great model, they're just presenting a great model in a very intentional way. That way happens to be drumming up its cybersecurity skills, but those skills are present in all their previous LLMs too.
If you run Project Glasswing with Opus 4.7 instead of Mythos, it still works, just not as effectively... or honestly, maybe even more effectively if you account for final token cost! Since they'll likely want to squeeze better margins out of Mythos than the workhorse models, Mythos might be so expensive that just getting un-moderated access to 4.7 and throwing in the same number of dollars worth of tokens at various codebases uncovers more vulnerabilities!
But the latter half of that paragraph is assuming you're outside Anthropic: Anthropic is doing all of this at cost, so obviously the best model they can muster is the best option to offer.
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The key is, Mythos isn't using scaffolding or and approach that no other model can meet the floor of. People jumped to small models and that's a bit of a stretch... but the best non-Mythos models can obviously be put in harnesses and used to find vulnerabilities at scale.
Part of the proof there is Anthropic themselves cranking up their cybersecurity request filters and going overboard with CC prompt injections.