If it was not possible to support the assumption with evidence - why include it in the report??
Their (ATSB) reports are usually pretty bloody good.... but this part....
The PIC’s exposure to a significant period of single-pilot operations during the year prior to the incident had the potential to have adversely affected his ability to operate optimally in a multi-crew, high performance aircraft. However, it was not possible to establish if that recent non-airline experience influenced the pilot’s actions on the day.
- not so good.