Hezbollah already has thousands of troops in Syria fighting ISIS. They also hold about 10% of the seats in the Lebanese parliament. Is this kind of cooperation really all that "unprecedented"? Seems like a the natural result of ISIS spilling out into Lebanon.
From what I'm reading, this also doesn't seem like your classic military defeat. It's sounds like a 3-way truce, where ISIS is agreeing to move back into Syria, and Hezbollah is agreeing not to attack ISIS anymore in Syria:
http://www.aljazeera.com/news/2017/0...042420298.html