The tutorial assumes that no other interrupt sources will "fire".

Timer0 interrupt hardware auto-vectors to the fixed address 0003H. At 0003H you can put your entire 8-byte ISR (filling the range 03H-0AH) if it's that short, without risk of overwriting the code locations that will be automatically vectored to for INT1, Timer1, etc. Conventional practice in a multi-interrupt environment is to have either an interrupt-quiescing stub of some sort or a jump (SJMP or LJMP) to the "real" ISR at the interrupt vector locations. These short instuction sequences avoid overwriting the space allocated to the other interrupts.