I guess that the flight might not be insured if the F/O had not passed their final line check, (or the training paperwork had not been updated to say they had), or if a training F/O was flying with a standard Cap rather than a TRI or TRE?
I can quite see how a mix-up could happen. The F/O, during their line training, would naturally assume the Cap was a TRE, so wouldn't ask*, or offer any information, unless it came out in normal conversation. And a non-TRE Cap would not ever think their F/O was a trainee. I have never known anyone check licences apart from a TRE doing a line check, or in the Sim.
*Wouldn't go down very well if they did ask to see the TRE's licence !!