There’s always something about these reviews that throws me off. If I’m playing an MMORPG, and am in the main city or a raid, playing at 4K, my GPU is capped at 100%, and according to reviews like this, my GPU is the bottleneck. But in these scenarios, having a faster CPU (like the 9800X3D) will almost always improve frame times, and provide a more playable experience than upgrading from a 4060 to a 5090. But going off this review alone, as a noob, I’d upgrade my GPU, and still have all the same stuttering I did before. Food for thought.
MMORPGs are a special use case, they're worse then even your run of the mill PVP FPS. Why? Well a couple of reasons.
There is virtually no graphical exchange between a player's computer and the game server. The server is a massive database containing 3 dimensional values for every location in the game world. Then there's other server/s with location data for every moving element like NPC's and players, where they are, where they're heading, and their speed. As well servers also keep track of the interactions between well... virtually everything.
So there's a lot of data flowing between a player's computer and a the game server/s. And this puts a massive load on the CPU to begin with. But it gets worse the more players are in any one area. Because not only are the server/s keeping track of everything, they also have to supply each player with information about every other player. How well things work has a lot to do with the Net code. You might have <30ms latency, but other players might have 50, 75, or 100+ ms latency and that will affect how well the game server can refresh what you see.
So how does a better CPU help? Well there's more resources available for all the non-graphical workload and this can free up more resources for graphics. It's the main reason why the article was looking specifically at single player experiences. Even your run of the mill shooter has the same problems as a MMORPG. It's why most have a hard limit on player count. Something the MMO designation doesn't allow or it wouldn't be a MMO.
There's also one other factor that has a minor effect. There's something called host processing, which simply means that most on-board "hardware" like your sound, or network are only interfaces. All the actually processing is done by the "host" processor, your CPU. In most normal use cases your CPU doesn't even notice the extra load they add. But remember how I mentioned that a MMORPG has a really high network load? So your NIC is adding a abnormal load to the CPU and more CPU resources will help mitigate it.
A perfect example of this is Ironforge in vanilla WoW. It was the only auction house in Azeroth, and due to that there was a massive amount of players there at any one time. So much so that it created all sorts of havoc. I can remember running just outside the bank one second and finding myself in the moat between the bank and the AH the next. Blizzard did a lot of work on the net code, but what really helped was simply making more AH locations to reduce the player count. Later they upgraded to phasing, so that the load was split between servers.