But in that sentence isn't there the implication that the editor of the dictionary thinks usually UPSET can help people improve their health?
There is no implication that the editor of the dictionary, or anyone else, thinks that upsets can improve health. You have to take 'not improved' as a phrase in its entirety. We use this type of expression when a desired outcome is prevented or postponed by something.
John's appearance was not improved by the large food stain on his jacket.
There can be a jocular nuance: that is to say, a kind of joke is being made to emphasise the point.
My chances of promotion at work will not be improved if I punch the boss.
The flavour of curry is not improved by the addition of coal dust.
The safety of the process was not improved by the storage of gasoline near to a furnace.
The speech was not made more intelligible by the frequent coughing of the speaker.
The atmosphere in the hotel room was not made more fragrant by the presence of a three days dead dog outside the open window.