k
Again, you are comparing Manhattan to Crown Heights (and presumably a letter to someone who doesn't consider himself a Lubavitcher, to words said to a חסיד of the Rebbe), and taking an excerpt of a letter and ignoring its context. Show us the entire letter (and preferably also the letter to which it is responding).
It seems to me that whoever this letter was written to, probably wrote to the Rebbe that he plans to have an eruv erected so that people can carry their tallis to shul. To which the Rebbe is responding that in his opinion that approach is wrong. The only reason to erect an eiruv (if feasible) is to save those that will carry regardless, from doing so באיסור. And in order to not get people that wouldn't carry without an eiruv, it should be done in utmost secrecy.
My guess is that if the recipient of this letter accepted the Rebbe's position, it is likely that he didn't go ahead with the plan.