NOT operator on Boolean in c++

I'm trying to use this Arduino code in my program, the LED will stay on for 5 sec then turns itself off

#include <elapsedMillis.h>

int led = 13 ;
elapsedMillis timer0 ;
#define interval 5000
boolean timer0Fired ;

// the setup routine runs once when you press reset:
void setup() 
{
 pinMode(led, OUTPUT);
 digitalWrite(led, HIGH);
 timer0Fired = false;
 timer0 = 0; // clear the timer at the end of startup
}

void loop()
{
 if ((!timer0Fired) && (timer0 > interval))
 {
  timer0Fired = true;      // don't execute this again
  digitalWrite(led, LOW);  // turn led off after 5 sec
 }
}

I don't understand how the if statement works inside the loop, !timer0Fired should evaluates to 1 but when I print it out it's 0, so when timer0 exceeds the interval the if statement should evaluates to be false, I tried it on a UNO and it works.