Sounds like that dude is smoking crack.
If he's talking about vehicle platform, there's no way in hell.
If he's talking about computer platform, I still seriously doubt it. The Mazda3 has been out for a few years already (just not here). Why would they change electronics?
Besides that, the Mustang comes with either a V6 or a V8. There's no 4 cyl option. Not to mention the whole FWD vs. RWD thing.
It just doesn't make sense. There might be a shared component or two but hardly an entire system, much less a platform.
I personally know a lot of Ford engineers and most of them are complete morons. Dumbest smart guys you'll ever meet. (crazy)