I do not think ACM article is very relevant here, because what is speaks about is, essentially, the fact that what we traditionally thought of "disks" is now getting as fast as RAM, because persistence is achieved via new technologies.

A machine with a measly 256GB of memory can do around half a million Argon2 hashes simultaneously

Not necessarily. There is always the limitation of memory bandwidth. Although, I do not see saturating that bandwidth as a design goal of Argon2.