I think the higher-ups in the NDP thought they could get many voters to strategically vote for the NDP by telling people that the NDP was the government-in-waiting since the NDP had 100 seats and the Liberals only had 35. But the NDP was obviously in third place and the voters weren't fooled.
That argument was always hopeless. It came out in extremis, the first time, I think, in that video from Brad Lavigne.
What surprised me is that both Peggy Nash and Tom Mulcair deployed it themselves. It might have had some small resonance if it had been used earlier, despite the speciousness of the argument. Trotted out as late as it was, it looked ridiculous.
I think it would have made sense to use it at the beginning of the election when the NDP was in first place. Using it at the end of the election when the NDP was clearly in third place probably sent votes to the Liberals as voters were aware of which party was in a better position to evict the Conservatives from office.