He means that if another plugin changes the damage received by the tank through hooking OnTakeDamage too, you may output the wrong results,
cause SM may execute your plugin first, and the one that changes the damage next.
I usually use the "OnTakeDamagePost"/"OnTakeDamageAlivePost" hook, but I'm not sure if this solves these cases
I already had a lot of problems trying to calculate things on Post hooks (not triggering functions or outputting different values like attacker)
I don't know if for the player entity you could deal with these situations, but with prop_physics I know this happens.
Still is a good code snippet (as always) and may work in most scenarios.