Employment in academia for doctorates w/o same field undergrad
Don't base life decisions on anecdotes. I'm sure such cases exist, and, other cases in almost any combination. But your future as an academic, or even in industry, will depend far more on your later work and far more than that on the potential for success that you can demonstrate to anyone in a position to hire you. 

But it would be a mistake to stay in Mechanical Engineering for life just for such a reason. It might be a reason to switch earlier, rather than later, but only so that your path into a physics doctoral program might be eased a bit. 

But, a single case is always a poor indicator as there are always, potentially, other, unstated, reasons for the outcome. 

Create your own life. But work hard at it.