Yes and no.  Without their gains in Quebec they would have only won a minority government so it was deciding in the sense it was what gave them a majority government.  Interestingly enough if you look at the most recent polls, it would be deciding in the sense if an election was called today results in the West and Atlantic Canada would probably be the same, but in Ontario the Liberals would lose seats (largely due to spillover from Wynne's unpopularity), while gain them in Quebec (little in the way of opposition there).  Now off course those are just the polls and what will happen in 2019 is anyone's guess.