Not the original commenter but my guess is that it is some kind of fine-grained CPU fingerprinting. Something like: http://s3.eurecom.fr/docs/ccs18_iskander.pdf

Oh, fascinating. I've suspected this is possible for a long time, but have never seen any ideas on implementing it.

I have a POWER9 desktop, and a fair share of other users do so for privacy/security concerns, pretty hardcore Tor browser guys and the like. I've mentioned before that fingerprinting firefox on ppc64le would be very easy because of timing the non-JIT'd JS engine. I guess there's potential for much more specific fingerprinting.