A) 770 was referred to by its address for years before the Rebbe spoke of the significance of the number itself
B) many other landmarks (such is apartment buildings) are referred to by their address.
C) as per the Baal Shem Tov’s teachings of hashgacha protis, the Rebbe's teachings are filled with lessons learned from what may seem to be insignificant.
So yes, it was השגחה פרטית that the address of the building bought in 1940 was 770, which not coincidentally is significant for reasons the Rebbe said.
Had the address been something else by השגחה פרטית, the Rebbe would certainly have found meaning and significance in that.