Are professors generally straight forward about admission decisions?
I take "I cannot make any promises" to mean ... I cannot [i.e., I won't] make any promises.  Behind that door could be anything.

In other words, it added no information to your decision about whether to respond in advance to School B's offer.

I wouldn't recommend asking the same professor again, but a more informative question for you to have asked would be when (with specific reference to the April 15th deadline that most graduate programs operate under) you would hear back.

Your best bet is to wait.