Thank you very much for the paper.

For a bold claim it would be better form to indicate how or at least with which search engine the search was done. But the claim is much smaller then the Guardian makes us believe. They do not say, that it is the first study, but that it is the only one with currently available products. Directly before this sentence they talk about the rising use alternative menstrual products, such as “menstrual cups, menstrual discs, menstrual underwear or reusable pads”.

I would expect that manufacturer with some RnD budget will do more tests than the ones they publish. But if their results are inconclusive or unwelcome as you said, they will not publish it. They might also not publish good results for simple marketing reasons (“Bigger number is better”), but their product might still contain improvements due to those results.