Personally: I am not a great fan of 5-Whys for problem solving... but even the adherents will recognize that there is no logical rationale regarding the number "5".
Obviously: the auditor should have issued the observation in such a way that the non-conformance should be well understood (by the CA team, by the org, by the third-party).
I don't work at the company, so what follows is speculation: Is it possible that the "root cause" is that titles on an org chart do not 100% align with job duties/responsibilities, such that for any given human with a specific job title there may not be evidence which demonstrates that human is trained to execute the job responsibility?
The argument that anyone with a given job title must be trained to do the assigned work doesn't pass the red-faced test for me, because if this was true a company ought to be able to make the case that no employee has to attest they did any work by name... they could simply write down their job title.
I don't want to propose action items to address this finding, but my guess is that (1) formally documenting specific training assigned to the humans, (2) updating the WI to indicate which training is specifically necessary to to execute any given WI, would be enough to erase any doubt that someone was ever trained, or trained at the time of execution. For a small company, I wouldn't expect you need to lean that hard into a training overhaul, but it doesn't seem unreasonable to document when individuals are qualified to execute certain WI.