More usefully, the reports are free to read, but the testing itself costs money, so coverage is not complete — only suppliers that enough people care about are fully tested.
A vendor can request a re-test if they believe VendorInvestigate made an error, and that dispute process is documented.
Cost per test is higher than for Janoshik or Medutest, but coverage of multiple vendors is cheaper than testing everyone yourself.
Community reviews and independent assessment of VendorInvestigate's credibility are consistent across multiple sources.
Favorable reports are valuable and should increase your confidence in a vendor.