Sample bias in the sense that they may miss companies in the S&P 500 that aren't as well reported on (possible but I would guess not super likely), or that fortune 500 companies are not representative of companies overall (certainly true but still a reasonable scope for analysis)? Or something other than that?

I know researchers especially junior researchers are very critical of others’ work. But they focus on s&p 500 because theses firms are big and closely followed by media.

People generally leave reviews because they are unhappy with a thing. That alone is sampling bias from Glassdoor. Rarely do people leave a review because they are happy with something.

Why is that a bias? When people are more unhappy, they post more negative comments. If they are less unhappy, then they post less negative comments. The bigger concern is that employers post fake positive reviews, which biases against the findings.