Quivering Palm vs Retaliation

Hi! I had this recent event as a GM where a Open Hand monk used Quivering Palm on my player's berserker. He failed his con save and dropped to 0, but he used Relentless Rage to make another DC 10 con save and stayed at 1 hp instead. Then he wanted to use his reaction to use Retaliation and hit the monk, who was currently at 5 feet of him. Retaliation says:

"Starting at 14th level, when you take damage from a creature that is within 5 feet of you, you can use your reaction to make a melee weapon attack against that creature."

The thing is that in that moment I wasn't sure if that counted as damage or if it worked more like a power word kill without the killing thingy. If he had succeeded the con save, I'll definitely count it as damage because I'd have to roll 10d10 necrotic damage, but with the "drop to 0" I'm wasn't sure. For the moment we have decided to count it as damage since it's a really particular situation, but I wanted to ask here if anybody has faced this before or if I'm just thinking too much of the wording and Quivering Palm's failed save is just a super massive damage thing and should be counted as that.

Thank you!