Mistakes were made.
"A bomb exploded near midtown" uses the active but tells us nothing about who was responsible.
If we care about clarity of agency, we should look for it, not poor proxies.
If we want agency to be clearly attributed, we will do better to ask whether agency is clearly attributed than to ask if the passive voice is used. It's what we care about, and it is a determination people are often better at making in the first place.
I do think it is being identified (at least) as an important or effective way of noticing when you are being evasive in your own writing - otherwise why is "the passive is often used to conceal agency" even relevant to the question of improving your writing? And I think to that end, asking the more direct question is better.
“The passive voice, too, has several uses in English. One of them […] is indispensable to classic style: the passive allows the writer to direct the reader’s gaze, like a cinematographer choosing the best camera angle.”
The example he gives -- and I'm sure you can come up with many more -- is,
See that lady with the shopping bag? She's pelting a mime with a zucchini.
versus,
See that mime? He's being pelted with zucchini by the lady with the shopping bag.
And there's this,
“As the linguist Geoffrey Pullum has noted, there is nothing wrong with a news report that uses the passive voice to say, ‘Helicopters were flown in to put out the fires.’ The reader does not need to be informed that Bob was flying one of the helicopters.”
We certainly don't need to care about the agency for every trivial matter, but we certainly should know when clever 'camera angles' are being used by the author to hide an agent in the shadows.