> Why did they stop and take no further action?

Simple: further action was not requested in the Jira ticket, and what is not in a Jira ticket is not getting done because the team responsible for doing the needful is already overloaded with other crap.

Or maybe because the higher-up whose authorization is needed to go on a few days worth of deep dive other than doing exactly what is asked and accounted for in tickets doesn't have the time for a few minutes to explain to them why it is needed, or the higher-up needs authorization from finance or legal first.

Obvious /s, but I wouldn't be surprised at all if this is exactly what happened. If I were to guess, the "legal" is my biggest suspicion - if a provider reacts on notice of illegal/harmful content, they're just fulfilling legal obligations. But if they go and actively wade through the archives to find more incriminating content, that might be construed as Github doing active moderation of their own, leading to a loss of pure content hoster legal protections.