Actually, having read the Ombudsman's ruling, it seems that the key factor leading to them ruling against the insurer was that the insured had used an aggregator site to provide his details.
The insurer asked a simple question in their proposal process: "Has the car been modified in any way?", but the aggregator had added an explanation that stated, "A modification is any change to the manufacturer’s original specification or features. That includes things like new stereos, body kits or spoilers, alloy wheels, new paintwork and any performance enhancement."
If the original question about modifications had stood unqualified, then my suspicion is that the Ombudsman would have had to find in favour of the insurer, but the supplementary explanation specifically mentions various things that are considered to be modifications and it's hard to see how PPF is analogous to any of those examples.
On a broader point, certain cars were always delivered from the factory with PPF in some locations. One example would be the PPF on the trailing edges of the rear doors and the leading edges of the rear wheel arches on the CLS. How could the "average consumer" be expected to know whether this was a modification or not?