The arson thing was pretty thoroughly debunked. "I would expect better of HN posters."
https://ottawacitizen.com/news/police-arson-unit-probes-otta...
"We don’t have any direct linkage between the occupation—the demonstrators—and that act." [1]
[1]https://www.cpac.ca/episode?id=fa5721d6-af8b-48d9-9848-73dcf...
What that likely means is that they don't know who the people in the video are. It seems pretty likely to me that it's related to the occupation, especially since there was a 'confrontation' earlier that day with residents of the building. It's not like buildings are burnt down in Ottawa every day.
No building was burnt down.