The double minus ( or double unary ) works like this, a single minus does the Math of changing the result to Negative, the second minus, changes it Back to Positive, any math done to a text representation of a number turns it into a Real Number ( like the *1 , or +0 )
For Excel, 0 is False, anything other than 0, positive or negative is True.
But that's not why +0 is used here, it's just adding 0 (zero) to coerce the text number to Real Number without changing it's mathematical value, like *1