The question is the problem vs a problem. This research uncovers a problem, it does not explicitly state what the problem is. What problem this research found is what people are questioning.
Most of the comments (all of them that I've seen) say that the effect is real, but the source isn't necessarily a bias in the reviews, because of how the study was conducted (eg, there could be a hiring bias).
Bringing up a different cause to explain an effect is not dismissing the effect as existing in the data, merely calling in to question the source of it.
This thread includes people's opinions that this is not a real effect, and is instead the result of poor methodology, sampling bias, and whatnot.
It is really not fair to say that the problem's existence is undisputed. There are plenty of people who dispute it, and that's also a problem.