It doesn`t reverbate far and wide because it is a poem. Especially it is not a great poem by literary standards.
It does reverbate because it is controversial.
And - as a lot of other people have pointed out - it is nonsense. Israel never threatened to destroy Iran - Iran has threatened to "put Israel off the map" more than once.
Israel has the atom bomb but never used it. I wouldn`t trust Iran not using it once they have it. Therefore I consider their nuclear program a problem.
Frankly I can`t understand why people who are against war, nuclear weapons and even nuclear power used for energy are neglecting the dangers when other-than-western-world-countries have them.
Back to Günter Grass: I don`t think he is antisemite. He is simply wrong here.
As for moral authority he ain`t one: I don`t blame him for being conscripted into the SS as a teenager, he was probably a mislead youth then. But someone who was always first of accusing others for being dishonest regarding their behaviour during the Nazi rule while he himself forgot to mention his own histotry for about sixty years can`t claim any superiority anymore.
This begins with the title "what must be said". Grass is claiming a singular moral position here. Apart from the fact that many right wing writers argue with similar patterns ("it must be allowed to say...") it is not in any way singular or heroic to critisize Israels politics, export of weapons etc. This has been done a thousand times or more, by politicians, writers, the press or here on the cat.