Synthetic vs Natural
the article states:
studies have found that repellents containing synthetic PMD mixtures are not as effective as those containing naturally derived PMD where the PMD content is in equivalent concentrations.
Reference #4 ("PMD, a Registered Botanical Mosquito Repellent with Deet-Like Efficacy") Never compares formulations with PMD in equivalent concentrations. They compare a 20% OLE product to a 10% synthetic PMD product. Hence this citation does not support the claim.
Reference #5 ("Green synthesis of para-Menthane-3,8-diol from Eucalyptus citriodora: Application for repellent products") actually finds that synthetic PMD is more effective than the essential oils and thus completely contradicts the statement here.