I disagree with this. As someone who has to deal with intense mud every single spring, thrush is a constant threat, as well as scratches and heaven forbid anyone gets injured because keeping it clean is next to impossible.
That amount of mud for any amount of time can be EXTREMELY lethal - regardless of it's a severe skin fungus, a badly infected wound or even colic from eating off that all the time.
I would argue that these conditions are almost as bad as it gets. A horse is not a pig, they are not designed to be wallowing in filth and feces without severe consequences.