Evidence limit
A multifactorial problem
Impaction means material has obstructed the gastrointestinal tract. Substrate may be involved, but particle properties, the amount swallowed, temperature, hydration, diet, health and individual feeding behaviour can all affect risk.
That makes both extremes unreliable: loose substrate is not automatically harmless, but finding substrate in the gut does not prove that the material alone caused the entire problem.
Evidence limit
What responsible advice looks like
Assess the complete enclosure and the individual animal. A substrate choice that is manageable in one well-controlled setup may be inappropriate in another.
Where direct evidence is absent, the honest approach is to explain the risk factors and identify any recommendation as experience or policy rather than claiming science has settled the question.