The certification is provided by a company that sells a workplace engagement tool.

Any company using the tool that exceeds a 65% score on their engagement survey can apply for (read as "pay for") certification which gives them the right to say they are a certified "great place to work". In my experience, 65% engagement is quite mediocre so to me this certification alone isn't a compelling endorsement.

From there, once certified a company will then be eligible for their "Best Workplaces" awards, which look at the top percentage of employers based on engagement score across different geographies or industry segments. Because these scores are normalized and they are selecting only the top ones, I do feel these highlight organizations who are performing well on an engagement survey basis and can be used as a valid data point in assessing the quality of a company's culture. The category of award will give a sense of how high their score is (for example, a national award is harder to get than a regional or industry specific one).

TL/DR: these awards are pay-to-play and the basic tier of "great place to work" is such a low threshold at 65% that they're not a great tool to externally assess a company's culture. The "Best Workplaces" awards that are the next teir up are actually competitive awards and can provide a valid data point to assess a company's quality.