No account needed. Choose the files you were given with the report. They are checked in memory and not kept.
It proves that the PDF and CSV are exactly the files Offboarding Proof signed (an ML-DSA-65 signature, NIST FIPS 204, with the key published at /.well-known/rootsystems-signing-key.json), that not one byte was changed, removed or reordered since, and how the report links to the one before it. With the .ots file it also shows whether the report's fingerprint is anchored in Bitcoin, and from when.
It does not prove that what the connected systems reported was true when they were read, that every system a person used was connected, or that the report's earlier history is complete. The company's full report history is checked on its own Reports page.