Wasn't it the insistence by some to essentially equate Imagism (big I) with symbolism that led Pound to start referring to Vorticism and to abandon Imagism. I think Pound was intent on showing modernism and its subset, vorticism (imagism) as very distinct from symbolism or any of the other modes of the romantics.
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The issue here is, as you note, what one means by "symbolic." Hughes seems to think it is the same as "symbolist" in the notion of being a gateway to a spiritual world outside physical reality. That would seem to cut out a great deal of poetry--like imagism, or WCW or Levertov or any poet who saw or sees poems as ways to engage with the material world directly.
Otherwise, as you say, it is just a tautology.