In fact, Mr Trudeau was speaking about the Stephen Harper government's inability to present a strategy to stimulate economic growth. Economists disagree with one another to various degrees on how much such growth leads down the line to less government expenditures. The theory is that increased government revenues through taxation rolls ultimately to a decreased demand for outlays to departments such as employment insurance and welfare. Part of the PM's platform was to support infrastructure building, which hopefully would get the ball rolling and help lead to the above.
The original quote:
"The commitment needs to be a commitment to grow the economy and the budget will balance itself."
In its proper context the idea doesn't sound so harebrained.