"The Lake Chad branch of (Boko Haram) is clearly willing to keep fighting ISWAP until it can speak directly with (Islamic State) on next steps," said Jacob Zenn, editor of The Jamestown Foundation's Terrorism Monitor publication.
The two groups have waged a bloody conflict in Nigeria's northeast since their split in 2016, sparked in part by ideological disagreements over Shekau's wanton killing of Muslim civilians.
Vincent Foucher, a Boko Haram researcher at the French National Centre for Scientific Research, said the cleric's comment that his group believes in Islamic State is a sign that Boko Haram may be seeking a form of arbitration with them.
Nigerian security forces could also exploit a rift between the two insurgencies.
"That's an opening for security forces to step up attacks now that they're fighting each other… and also perhaps an opportunity to divide the ranks, to say, 'Those of you who want to defect, defect and come to us'," said Bulama Bukarti, a senior analyst with the Tony Blair Institute for Global Change.