By Agence France-Presse
The video was found in a USB key belonging to Cherif Chekatt, 29, who was shot and killed by police after a two-day manhunt in the city following the December 11 attack.
The Islamic State group claimed Chekatt was one of their "fighters", but this was dismissed by French Interior Minister Christophe Castaner.
Chekatt's father, Abdelkrim, had said his son believed the Islamic State "fought for a good cause".
"I always told him they were criminals," his father added.
Four of the five people killed in the attack were foreigners or born abroad. They included a Thai tourist, a Franco-Afghan garage owner, an Italian journalist and a former Pole who had lived in Strasbourg for the past 20 years.
The dead also included a 61-year-old Frenchman who had just retired and was shot in the head as he stepped out of a restaurant.