[2020] Is this a normal interaction or a bug?

One of my friends just got confused because of the weird interaction of damage and planeswalkers.

He casted [[Storm's Wrath]], then activated [[Sarkhan, the Masterless]]'s +1. Then the planeswalkers just died because there were 4 damage marked on them.

But according to cr 120.3, only damage dealt by a source with neither wither nor infect to a CREATURE will be marked, for planeswalkers cases, damage only causes removing loyalty counters. But when the damage was dealt, Sarkhan and other planeswalkers weren't creatures.

Screenshots here: https://imgur.com/a/H9cZRnR