First of all, I wouldn't trust anything that Putin says. Keep in mind, this guy managed to steal a Superbowl Ring from the Patriots owner. And, he's done a lot worse than that, too.
Secondly, the idea of destroying asteroids with missiles may not be effective. In fact, it may make the situation a lot worse. The problem is that asteroids can be made up of different substances and be of different sizes. Some are merely a collection of rocks strewn together by gravitational pull. Blowing up means you will have hundreds more asteroids heading for Earth.
Scientists in Russia probably know that. Thus, he's probably using it as an excuse to deploy these missile (and I'm just using "probably" just on the outside chance that he is actually going to use these missiles for that -- After all, he is unhinged to some degree).