Celsius and other energy drinks have not proven to cause "brain fog." This is also very difficult to measure, because "brain fog" is not a definitive term, and there is no real way to diagnose this. The article you chose also does not mention Celsius specifically, but caffeine in general. There is no doubt that ingesting large amounts of caffeine can have adverse health effects. But in my research, I did not read anything regarding "brain fog."
https://health.clevelandclinic.org/are-energy-drinks-bad-for-you
This article discusses the negative health effects of very caffeinated energy drinks, like Red Bull, etc. These health impacts include anxiety, depression, diarrhea, headaches, nausea, stroke, high heart rate, etc. Nothing mentioning brain fog. This source is a very credible online health source.
The only sources I found saying that Celsius and other energy drinks cause "brain fog" was TikTok, Safari AI overview, and that is it.