Let's set aside that it was set up that way on purpose to calm investors as Chapek was considered too raw to handle a company as massive as Disney in that situation (heading into Covid). We can also ignore that Iger did leave completely after his allotted "mentoring" time was up and was reduced to grousing about things in the media.
That would still leave your theory of events as a massive gamble for Iger. If Disney does fine under Chapek, Iger does not come back. Period, end of story. Instead Chapek screwed up in every way possible. He went against the board on purpose, he p-ed off the street (big no-no for a CEO), he p-ed off the workers, he tried to lie on an earnings call, started a fight with an entire state due to his incompetence, paused projects, redid others (making them worse), canceled still more. Turned the whole place into a streaming factory with an accountant in charge of creative decisions, etc.
Don't get me wrong, Iger isn't some innocent bystander, but he did not make Chapek do any of those things that lead to his return. If Chapek isn't a disaster, Iger is still sitting at home today.