According to the article, Singh is only saying the NDP will leave the supply and confidence pact. That doesn't mean they'll vote against the government on a confidence motion.
That's good for the NDP, come next election. They get to say they humanized the Liberals policies, but they withdraw support when the Libs didn't hold their end of the bargain.
The Liberals are plummeting in the polls, so Singh is probably hoping to pick up some disillusioned Liberal votes next election.