Consider that lots and lots of people are naturally going to be curious about the new Tomb Raider game when it releases. They're then going to investigate PS3 and PS4 versions, only to discover that they don't actually exist, which is a pretty big talking point, as we've seen. This topic of conversation then snowballs and publicly reinforces the statement that Microsoft is serious as a heart attack about Xbone and securing exclusives for it, which makes buyers feel more comfortable about supporting it and receiving future cherry exclusives.
It's basically taking advantage of the high profile branding Tomb Raider carries to get people talking about Xbox, and buying Xbox hardware, two birds with one no doubt expensive stone. And if they keep doing stuff like this, it'll probably pay off, at least in terms of re-establishing their hardware.