What about it makes it a "modern" replacement rather than just a replacement? Is there something about Redis and Memcached that is "outdated" in the (relatively) short time span they've existed (compared to something like C)?
In practice, software architecture is forever. It is nearly impossible to change the fundamental architecture of software people use because applications become adapted to the specific behavioral quirks of the architecture i.e. Hyrum's Law, which would not be preserved if the architecture was re-designed wholesale.
But yes, Redis is very much designed against the grain of modern hardware. It also is a very minimalist design, that works well within its limits, but falls down hard when you push those limits, particularly with snapshotting and replication.
Modern is used here as a selling point