alt Hacker NewsArticle does not mention the other reason: in the interview with Dwarkesh, Amodei remarked about how other organizations are copying or training off Opus for their models.
By delaying allowing others to train off Mythos, they hold their SWE-Bench Pro head start longer so among other things, the USG can't but notice Anthropic's lead when they're deliberating on whether to further substantiate the "supply chain risk".
Good point.
Precise motives are hard to work out as a general rule. Ultimately, it often comes down to a decision that decision makers like or don't like for a confluence of reasons.