No you don't
The scenario is this: the signal one is trying to debounce fires the interrupt - let's call this the "button interrupt". The button interrupt gets disabled, but other interrupts do not, so one can delay() inside the ISR to do sw debouncing. So while the "button ISR" is running, timer0 fires. And its ISR gets executed.
That's a nested interrupt scenario, isn't it ?
(btw, I'm not the OP).