I thought it was spelled out quite clearly in TFA: "GFNI".
GFNI Was only added to AMD CPUs in Zen4, basically the "just released" server CPUs (codename: Genoa) which are currently quite difficult to procure, even 6 months after announcement.
I don't see how that explains the encoding:reconstruction perf relationship. Looking at the first pair (12+4 encoding), epyc is about twice as fast as avx2-intel and four times as fast as graviton at encoding. But in reconstruction its two to three times slower than either.
GFNI Was only added to AMD CPUs in Zen4, basically the "just released" server CPUs (codename: Genoa) which are currently quite difficult to procure, even 6 months after announcement.