Do you also think this paragraph about a Manhattan Eruv was written by the Rebbe in hyperbole?!?
I'm not seeing the full letter here, but there no indication from this quote that the Rebbe is suggesting to make an Eruv with a condition of secrecy. On the contrary, the Rebbe is saying that even if you were to prove that it is feasible to make an Eruv according to Din, even then the absolute necessity, is that it needs to be done in secret. The word necessity is being used by the Rebbe on the part of secrecy, not on the part about building the Eruv in the first place.
It's possible that these people were going ahead with it anyway, and the Rebbe wanted to make sure at least this condition should be met.
I don't see a way of proving from this small quote, that the Rebbe is suggesting in any way to make one in the first place. Perhaps if the rest of the letter is published, we can see the context better.