That's not how tax breaks work. The government isn't giving money away, it's just pledging to not collect money in the future. I don't know the specifics of this deal, and $15B seems like a lot compared to the scope of this project, but it's likely that if they didn't offer tax breaks at all, this would have been built in Mexico or USA instead. There was probably a number in between 15B and zero that would have still brought them to Canada though.

Presumably the government accountants who made this deal expect this project to bring in significantly more then $15B in taxes over a decade or so of operation.