Negating the 'is' operator with a variable only works with '!' but not with '== false'

Why does (expr is type varname) == false gives a compile error, but !(expr is type varname) compiles?

public static void Foo(object o)
{
    if(!(o is string s))  // <-- Using '!'
    {
        return;
    }
    
    Console.WriteLine(s);  // <-- OK
    
}
public static void Bar(object o)
{
    if((o is string s) == false) // <-- Using '== false'
    {
        return;
    }
    
    Console.WriteLine(s);  // <--Error: Use of unassigned local variable 's'
}

Live example: https://dotnetfiddle.net/nYF7b6