From Chapter Fourteen of Rabbi Shloma Majeski’s Likkutei Mekoros, Volume 2. (Underlined text is the compiler’s emphasis.)
Translated by Boruch Merkur
Since Eliyahu must come before the advent of Moshiach (see Malachi, end; Eruvin 43b; Yad HaChazaka Laws of Kings 12:2) analysis is required as to how Bar Kochba was erroneously assumed to be Moshiach [even prior to Eliyahu’s arrival]. We may assert that in the sayings of our Sages the exact day of the coming of Eliyahu is not clear, whether it is 1) prior to the first appearance of Moshiach, 2) before his victory in war, or 3) prior to the acceptance of his rule by the masses, or the like. Thus, there was sufficient rationale to proclaim that Bar Kochba was Moshiach.
(From the handwritten manuscripts of the Rebbe Shlita;
Toras Menachem, Hisvaaduyos 5710)