I'm actually going to disagree with that statement. I spent all yesterday morning poring over reviews and graphs. From what I read, at 4k resolution, most systems are GPU bound, and upgrading the CPU hardly made any difference. This is exactly where I'm at right now - I'm gaming at 4K, and my 3090 is capable of pushing out 50fps. Upgrading to a 5800X3D would not change this, as I'm GPU limited at 4K.
I believe the real benefit is for somebody gaming at 1080P (or 1440P) who wants as many fps as possible, because they're running a high refresh monitor. In that situation, what I found from reviews is that the 5800X3D DOES provide a significant benefit in the form of increased FPS.