I think some are a bit too focused on job titles here.
CEO roles can vary quite a lot depending on the how the board separates the tasks. I expect Ajax rely heavily on the CFO for the running of the business. There will obviously be an experienced marketing person in there too, and others. I would guess that the majority of the "day to day running of the business" tasks are taken by others, free'ing up Edwin to focus on the football side of things, which he obviously has great experience of compared to most board room directors.
I wouldn't say it would be a bigger job to step into United as technical director or director of football. Obviously there is way more responsibility as a CEO, and being the figurehead of the entire company. But maybe... it's not quite a gigantic step down as "CEO to technical director" sounds like at first.