No third-party auditing could exist that proves you only have RAM in the system and don't have a secret disk in there with a magnetic reed switch in-line with the SATA power cable such that without sticking a magnet on the case the disk doesn't show up. Or that you aren't booting off a USB drive that you plug in only after the auditors leave.
Third-party audits are a scam to begin with and don't prove anything.
A third-party audit can prove that the system functions as shown without a hard drive, and a third-party auditor can, using contractually-authorized random unscheduled spot checks, physically inspect the live deployed servers to confirm the absence of any disk media.
Third-party audits prove something. They don't prove everything.
If they claim that they possess no data, and after some years and some nontrivial attempts no one has succeeded in extracting data from them, that is not nothing, even if it is not proof.
Third-party audits are a scam to begin with and don't prove anything.