Presumably endpoint detection & response (EDR) agents need to do things like dynamically fetch new malware signatures at runtime, which is understandable. But you'd think that would be treated as new "content", something they're designed to handle in day-to-day operation, hence very low risk.

That's totally different to deploying new "code", i.e. new versions of the agent itself. You'd expect that to be treated as a software update like any other, so their customers can control the roll out as part of their own change management processes, with separate environments, extensive testing, staggered deployments, etc.

I wonder if such a content vs. code distinction exists? Or has EDR software gotten so complex (e.g. with malware sandboxing) that such a distinction can't easily be made any more?

In any case, vendors shouldn't be able to push out software updates that circumvent everyone's change management processes! Looking forward to the postmortem.