That is definitely false - single core performance still matters a lot in many ML applications, where utilizing many (>8) cores is still difficult.
Is there an alternative RAM database that you like better that is multi-threaded?
Single-treading is one of those. There are times when having more than one thread to help process things would come in very handy, though I recognize that the cost of adding this can be very high.
It's something that will have to be addressed eventually for a single Redis process to take advantage of newer hardware with very low ceilings on CPU power, but huge numbers of cores.