The MSE POPLA appeal you have linked to does not apply in this case. That was for a PCN issued on land under statutory control, Stansted Airport bylaws where there can be no keeper liability.
It's up to you how you appeal to POPLA. Had Tusker and the employer carried out the correct procedure and an NtH been issued to the employer company and had they provided the employee authorisation to deal with it in the company name, then it would be an easy win at POPLA because there would be no keeper/hirer liability and as long as the driver was not identified, that would be the end of the matter.
If you want to try POPLA, then I really don't know as what the employee is appealing as. If they are appealing as the driver, then there is little to appeal on. If the are appealing as the Hirer, then as long as POPLA don't query the fact that the appellant is not the named Hirer, there is a chance it could be successful, again as long as the driver is not identified.
If they are appealing on behalf of Tusker under a letter of authority to deal with it in their name, I don't see any successful appeal that can be made, whether the driver is identified or not.
From the information provided, there has only ever been an NtK issued. Transfer of liability has not been correctly carried out according to the requirements of PoFA and so, the Keeper (Tusler) is still ultimately liable. If POPLA don't accept whatever appeal you submit, MET are then going to go back to Tusker and demand that they pay the alleged debt. Tusker will simply pay it, charge back to the Hirer and the Hirer will dock the wages of the employee.
In the final event, if the employee wants to recover that money, they will have to sue Tusker or get their employer to sue Tusker.