FFXIV, like most video games, only uses a handful of cores to run, and assuming Intel cpus or AMD TRs operates in a similar fashion to my Ryzen 9 3950x, it knows which of the cores on the chip are the "best" cores, and those are the cores that get the single / low threaded workloads. Me telling my processor that it can't use the 8 "worst" performing cores for this game won't change anything because it isn't using them anyways.