The fact that there's no intended flight backs his point of no certificate required. Since there's no federal prohibition (nor explicit allowance), we're left looking at lower jurisdictions.
Since there are thousands of airports and airport authorities that could create such rules, it wouldn't be unreasonable to assume some airport somewhere has a local prohibition on non-authorized taxi operations, however they so choose to define authorized. The only prohibition at my base is no (non ops) motor vehicles other than aircraft on the runway or inner taxiways.
If nothing else, AC 150/5210-20A, seems to allow for such local rules:
It then goes on to offer some suggestions and topics for consideration.
https://www.faa.gov/documentLibrary/media/Advisory_Circular/150-5210-20A.pdf