if the FAA 61:75 licence is issued on the basis of a basic UK or EASA PPL with no night or instrument qualifications then the FAA licence is also only valid for Day VFR Only. As ratings are added to the UK/EASA PPL then you can apply to the FAA to have the privileges applied the FAA 61:75 licence.
To clarify: you can obtain an FAA licence in this way and then add an FAA IR to it by doing the required training and passing the theory and flight tests. If you do that, you can then fly an N Reg in Europe and exercise instrument privileges.
Note: I am not saying that this is what happened in this case as I don’t know either way.