Red blood cell capacity of modern menstrual products: considerations for assessing heavy menstrual bleeding

No study exists comparing the capacity of currently available menstrual hygiene products using blood.

They don’t have to explain how they know. Literature searches are standard, and done before doing research like this. Funders want to know if they’re wasting their money on a question that has already been answered, and whether the proposed methods are appropriate given what has been done, and learnt, before.

That’s not to say that all literature searches are perfect. You can check on PubPeer for any howls of anguish from unacknowledged researchers. But the only legal requirement for testing is tampons due to toxic shock syndrome and its relationship to absorbency. It’s really unlikely that manufacturers are doing the tests without being forced to and, if they have done any, really unlikely they would fail to publish their results if they liked the results. If they are suppressing unwelcome results, the research might as well not exist.