Death is a weird thing to diagnose, and I doubt we'll ever have a perfect way of making such a determination.
For example, there are living, breathing people in the world this very moment who have no pulse. No, not even just a very shallow, difficult to detect pulse (which is itself already problematic), but zero pulse, because their blood is being circulated via a "water screw" style pump they had implanted to assist or replace their hearts that do not actually operate properly or at all anymore. Turns out as long as the blood is flowing, it largely doesn't matter if it's doing so in pulses or in a continuous stream.
People have "woken up" from being "dead" many times. A stopped heart can restart. A stiff and cold corpse can warm and stir. Oftentimes we help the process along in various ways, but it also occurs somewhat less frequently on its own.
Now, it's clear from the story that the hospital in question cut some corners and was negligent, I have no argument against that, so rest at ease. But this is such an unusual and bizarre situation thatI feel it's worth expressing 1) just how hard it actually is to be certain someone who has suffered a non-violent "death" (especially someone overdosed on drugs!) is actually dead and 2) how despite this fact, this sort of scenario is absurdly rare, especially because the difficulty in positively diagnosing death drives professional medicine to be very sure about death before making a final statement.