Where are the sources to back that up though? (Talking about the article, not you. Although if you could provide some historian's assessment I would be interested)

I imagine not the uniqueness, but simply the aggregation was its most attractive attribute at the time. Knowing another copy exists somewhere across Europe isn't really helpful if you had to travel months to retrieve it. Everything was there, ready to be consulted.

The arson is, as far as I know, an event that has been continuously recalled and depicted throughout history and, while it has most likely been romanticised at least a little bit, I struggle to believe it's a myth. Both the library and the fire are important symbols, especially in a time where we are seeing similar attacks on freely available knowledge