Having a 19% stake in selling 6000 planes (as the article mentions), is worth a lot more than a 49% share in selling 300 planes. Also, Bombardier had to constantly discount the plane and take lower margins just to get airlines to buy, which with Airbus's sales and support network will no longer happen. Plus a lot of the uncertainty of the project has magically gone away.
This is a great deal for Québec.