probably the latter, since all of these routers were unpatched, out-of-date routers, and that's how they were exploited in the first place.

however, the article specifically states that the court documents are all redacted when it comes to the details

It's not entirely uncommon for the latter to happen. Some greyhats have done similar things to clear out botnets in the past. It still counts as unauthorized access to a system though so most avoid doing so even if the intended result is beneficial

The U.S. has a very robust hacking capability, we just don’t advertise it and we concentrate on shutting down or infiltrating critical infrastructure in times of war or espionage.

Instead of hacking China to steal industrial secrets, we hack them to see if we could say open or close all the floodgates at the 3 Gorges Damn… China hacks us to steal state and industrial secrets, though they are now starting to focus on infrastructure.