I agree that it sounds curiously redundant. The "of us" could be removed from any or all of these sentences, and the remainder would convey essentially the same meaning:
We are, all of us, in the grip of various philosophical assumptions, presuppositions, hypotheses, even theories, about the way the world is, and about what matters in life. (M. Rowlands)
We are all of us bound to work toward this end. (T. Roosevelt)
We are sculptures and recordings and movement and canvas stretched, and we are all of us works of art. (K. Curren)
I suppose the redundancy is intended emphasize or reiterate that whatever follows has some universal or all-inclusive quality - although the scope of that inclusiveness depends on the context:
Although our experiences are different we share a common heritage of oppression: we are, all of us, women. (Z. Dé Ishtar)
I would prefer to offset the "all of us" with commas, but as FumbleFingers pointed out, that seems to be a matter of personal preference.
Out of curiousity, I repeated variations of the "we are all of us" search, using "we will all of us" and "we are both of us," to see if those phrases were also found. Both turned up plenty of results, mixed with and without commas (and one made a rather humorous parenthetical exclusion to the inclusiveness of the verbiage):
We are both of us nice of temper; we are both apt to kindle, and warm of resentment. (H. Godwin)
We are, both of us, perfectly capable of taking all sorts of chances. (J. Katzenbach)
By then it will be too late. We will, all of us, have made our
fortunes by then. (F. Norris)
My own prejudice is that we will all of us (except, let it be quickly
admitted, personal injury lawyers) be better off under no-fault than
under traditional tort liability. (H. Ross)
It's bad grammar. In point of fact, they must, as you say refer to very few people, which gives a strained but just about comprehensible sentence. What the author evidently intended, however, is people or many people. Probably the best wording would be to leave they as it is, and replace very few people with people rarely.
Best Answer
‘To be matched by’ does indeed, as you say, mean to be on the same level as (see verb, def. 2).
In the phrase X is matched only by Y, the meaning is that the only thing is equal to X is Y. In other words, to paraphrase your example:
Whether it is true that the metaphor used by Martin Luther King, Jr., in his famous “I have a dream” speech is ‘anodyne’ (deliberately inoffensive and placid) is of course a matter of opinion; but the writer of the article clearly seems to think so, because he is specifically saying that even though the metaphor is as well-known and ubiquitous (found everywhere) as it is, it is just as inoffensively placid as it is well-known. In other words, he thinks it is extremely inoffensive and placid.