Illusory troop movements and deceptive tactics date back centuries. Genghis Khan would have each of his troops set multiple fires at night to convince enemy forces his own were much larger. if you want them to overestimate you and not attack (or at least suffer morale loss), light more fires. If you want them to underestimate you and attack without being prepared, hide the fires.
While I much prefer to take a peaceful political approach to the problems facing the world today, I believe this tactic could be employed in modern warfare. It's possible that the opposition forces would have the technology to determine that the alluded to army was in fact fictional, but if they did not have the technology to do so, then it could be a possibility.
Even if the British were able to draw German forces away from invasion targets using a "fictional army" in WWII; that does not mean it would work in today's world. With the advances in military technology a feat like this would be nearly impossible. This would not work in many circumstances.
The military is much more equipped with technology that it was back then. They will be quite capable of researching and detecting sources of misinformation and misdirection. The "militaries" of the world are less prone to acting on impulse in light of the global implications/ repercussions it can have on their countries.