This makes no sense. Signing replacement players is near trivial, and owners can aproach players before the impasse is declared. If you declare impasse is say May, then its in court and decided BEFORE the season starts. Since there is a VERY good chance that the NHL will in fact lose, they will want to ensure they declare impasse at the last possible second.
I also don't know if the NHL will go down the impasse route. Impasse doesn't mean the owners win, but it does basically put an end to the proceedings, one way or another. However, if the owners think they can get the PA to completely cave next January, they very well might not declare impasse at all. It is a less risky strategy to not declare impasse than to declare impasse.