To be fair, that participant "commented", not "complained". And that same "comment" has been made regarding several other web browsers also.
Seems to me that it MIGHT BE something specific to that participant's hardware or self-admitted MODDED graphic's driver. I was unable to witness this "brightness" difference in one of the participants other-browser comments.
Yes, the fonts were "wider" in that other-browser comment, but to me the were the same "brightness" via screencap RGB, hue, saturation.
It's simply a "comment" (just like any of our "comparisons").
Yes, the fonts are "different", Yes, dark mode is always a NIGHTMARE in "my opinion".
But all is relevant to the "discussion".