I actually called to ask about this. I was told that no matter what the coupon says, it's only good on how you earned it. I repeated several times, "but it says Acela" and she just kept repeating back to me, "it is only good on the type of train you earned it with." If that's the case it is very poorly worded. I looked at last year's coupon and there was no mention of Acela.
I will actually be booking a trip in the next day or two and we'll see if they allow it for Acela but if the rep was correct, it won't work. It's all very strange.