I'd argue that if you buy an Intel or Nvidia GPU when you want to run Linux, you are actively sabotaging yourself. Clearly those two don't make good Linux drivers as the performance discrepancy is way larger compared to AMD/Radeon. For radeon the article says:

Ubuntu was 8% slower than Windows

(Leaving out raytracing bumps that to a 5.7% difference btw.)

Which seems ok to me. Especially considering that they mostly tested newer titles. If you check older titles, many of them perform on par or better on Linux than on Windows. For example notoriously World of Warcraft.

Quality can also be percieved by frame stability. 1% lows for example, which manifest in occasional hickups. While the average fps might still be equal, the game might feel less laggy if the framerate is more consistent. Which was not measured here at all. Other sites benchmarks (again for older titles) show less hickups iirc.

So the statement can be true depending on what you play and how you measure. It's very difficult to make a study that is unbiased and reflects the real world experience of most people. Some will have great results and talk about them online. Others will have bad results and talk about them online.

Bonus edit:

On windows I remember it starting a background update while I was playing an online game and it started lagging, even though the fps stayed the same. And there was no way to prevent these background processes from running when I was playing. This would not be captured in such a benchmark as well, because they update everything before running the benchmarks. But it would frequently happen in real life use. For me this was the killer UX difference that made me fully move to Linux 9 years ago.