Moshiach Memes

I was reading R’ Joseph Telushkin’s biography of the Lubavitcher Rebbe and he notes that in the 1950s-60s the Rebbe was vocally in favor of prayer in US public schools (and consequently opposed the 1962 SCOTUS decision Engel v. Vitale), in spite of the fact that such prayers were inevitably Christian in nature and would mean any Jewish public school students would have to pray with Christians. I was always told that Christianity is avodah zara/idolatry and that reciting Christian prayers or praying with Christians (even in a supposedly “non-denominational” manner) would be very bad, so how did the Rebbe/Chabad justify this stance?

Note that this is in no way intended to defame the Rebbe zt’’l, a man of whom I have much admiration, merely an attempt to understand this stance.