Most likely?
Never seen a necropsy come back saying "most likely".
There are numerous causes for impactions - most have nothing to do with substrate. Substrate caused impactions are so rare that most vets who actually encourage necropsies state that they never see them. In fact most impactions are secondary complications that are preceded by a primary infection cheesing up the gastric system............
Case in point, I sold some Louisiana pines to a lovely lady in Idaho. After a year of ownership two of them died from what she claimed were impactions.
Now toss out the coincidence that TWO snakes died within the span of 10 days and toss out the fact that her vet was too stupid to look at other potential issues and instead was willing to collect her $125 for treating the animals and claim they died of "impaction".
When a third snake in her collection died shortly after (a milksnake if memory serves) I pleaded with her to get the animal necropsied. The necropsy showed that the gastric tract was completely filled with ricotta and further histology showed a viral infection. Her bone dumb vet was willing to chalk up a third death to impaction when she had a virus raging through her collection, which was later tracked to a recent arrival.
Now there are a subset of hack vets out there who will willingly tell you something based on a WAG. As long as people continue to THINK that their animals died from impacted substrate and as long as vets are willing to take guesses on causes of death without recommending a necropsy; the stupid, useless, yet apparently persistent myth that feeding on loose wood fiber substrate is a risk will continue to pollute these forums.