In the Navy, one can be advanced in rank in a "frocked" status meaning that they are awarded with wearing the uniform and insignia of the higher rank, and the responsibilities this higher rank entails, but they will still collect the pay of their lower rank until they are no longer frocked. This can take several months.
If a PO1 (E6) gets selected as Chief (E7) and goes through Chief initiation and finally acceptance, he is allowed to wear the uniform of a Chief and to call himself a Navy Chief. However, he may still be getting paid as an E6 for several months. He may even get an ID that says (confusingly) CPO/E6.
Now, let's say he made Chief at 19.5 years but still had plans to retire at 20. He could call himself a Chief at retirement and wear the uniform of a Chief. He had been accepted as a Chief by the CPO Mess. But he will not be paid as a Chief and his retirement pay would be that of a First Class Petty Officer.
Is this stolen valor?
To be honest, I don't know all of the details surrounding Tim Walz. I'm just presenting a scenario where someone could conceivably, and rightly, refer themselves as a different rank than the one they retired at.
Ditto, I was frocked and it took 6 months to make it official.