Sure they can sue. What's your point?
(We've already established that it's notoriously difficult to prove a predatory pricing claim in court.)
As far as I understand the "traditional" test for predatory pricing is not defined in the law, it's merely how the courts decided to evaluate the cases sofar. It might be that the courts never encountered a case similar to Amazon's.
My point is that if Amazon is tested the "traditional" way, in my opinion there is little chance for the predatory pricing claim to stand. But Apple could try to convince the courts that (as you argued) the test needs to be done differently because it does not cover the practices Amazon is using. If successfull it could mean a completely different scenario.
I still think the courts would use the "traditional" test and not find Amazon guilty of predatory pricing, but with these matters you never know and I don't see much to lose from Apple's side compared to what would be at stake for Amazon, that's why I said that to me it should be a no-brainer.