Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Calculation/Logical.php
public static function NOT($logical = false)
    {
        $logical = Functions::flattenSingleValue($logical);
        if (is_string($logical)) {
            $logical = strtoupper($logical);
            if (($logical == 'TRUE') || ($logical == Calculation::getTRUE())) {
                return false;
            } elseif (($logical == 'FALSE') || ($logical == Calculation::getFALSE())) {
                return true;
            }

            return Functions::VALUE();
        }

        return !$logical;
    }
public static function NOT($logical = false)
    {
        $logical = Functions::flattenSingleValue($logical);
        if (is_string($logical)) {
            $logical = strtoupper($logical);
            if (($logical == 'TRUE') || ($logical == Calculation::getTRUE())) {
                return false;
            } elseif (($logical == 'FALSE') || ($logical == Calculation::getFALSE())) {
                return true;
            }

            return Functions::VALUE();
        }

        return !$logical;
    }
NOT.

Returns the boolean inverse of the argument.

Excel Function:
=NOT(logical)

The argument must evaluate to a logical value such as TRUE or FALSE

Boolean arguments are treated as True or False as appropriate
Integer or floating point arguments are treated as True, except for 0 or 0.0 which are False
If any argument value is a string, or a Null, the function returns a #VALUE! error, unless the string holds
the value TRUE or FALSE, in which case it is evaluated as the corresponding boolean value

@category Logical Functions

@param mixed $logical A value or expression that can be evaluated to TRUE or FALSE

@return bool|string the boolean inverse of the argument