Possibly game specific, as opposed to all games? The 9600k has a much higher base clock, which possibly really helps, but overall for most games the 600mhz difference in max boosts clocks, would literally mean anywhere from 5-10 fps difference between the two CPU's, like 10%, at 1080p.
I'd defo agree the 8400 is a budget-midrange CPU. It just so happens at 1080p it beats the pants of a lot of Ryzen 2xxx CPU's and is very competitive with even a 9600k.
I hate posting videos for comparison, but hey, it's Friday
:https://www.youtube.com/watch?v=R4ycNeKM6Nk
The difference between the CPU's FPS wise is nearly 100% due to faster turbo speeds, and IPC increase of the 9600k. With the same number of cores/threads. It's not about a bottleneck really. IMO.