I’m not an audio expert so this is just the general gist - but I think the reason it is affecting some headphones and not others has to do with resistance. Usually the nicer and larger the pair of headphones the more “resistance” it has which roughly means how much power required to drive the headphones and how sensitive they are to fluctuation in that power.
The higher the resistance the more power required and the less sensitive they are to fluctuations in power. Earbuds are definitely less than 50 ohms. The WH-MX3000s I have (a pair of regular Sony noise canceling headphones) are around 47 ohms so I noticed the noise in those as well.
I have a pair of Sennhesier 6XXs which are giant, great headphones, and they have a resistance of a whopping 300 ohms and I can’t hear the issue in those. But most headphones are closer to the under 50 range.
Your Hyper-X’s are probably somewhere in between the two.
I wonder if they were testing with headphones with higher resistances, or testing them in a situation where the GPU was at a lower clock speed and it was harder to hear the interference.