Footnote 35 on page 395 of William Braude's English translation of Pesikta d'Rav Kahana says, "See MhG Gen., p. 880, where an anonymous teaching characterizes all prophecies predicting redemption as no more than a pack of lies."
First of all, there is no p. 880 in Midrash HaGadol on Bereshis, as there are only 772 pages in that book, as can be seen from here.
Secondly, can anyone who has access to Midrash HaGadol check to see if it actually states this?