What was the plan here? Were they retarded enough to think the Sultan just... wouldn't come back to lead his troops?
Pretty much, yeah. Back then, it was expected that, once a ruler stepped down from its throne and choose a successor, they would permanently stay behind the scenes and do only the minimum to support the new leader. No matter the path the new ruler would take or which policies were implemented, the old ruler would respect that decision and at most would do some scheming or social maneuvering outside of the public eye.
Getting back and openly taking the leadership back like the Sultan did was not only very unorthodox but also politically dangerous: it proved that the successor was wholly uncapable of fulfilling his duty, that his counselors were inefficient or useless, and that the Sultan made a shameful mistake in choosing him as his heir. It was a damaging decision that would've raised doubts about the future of the empire and could've gravely damaged the image of the Sultan even if he delivered overwhelming results.
Hence, the confidence of the Cardinal at suggesting breaking the truce: the hungarians were absolutely expecting to fight an army led by a 12 year old boy and the thought of the Sultan coming back never even crossed their minds.