They very well could have used the same FPU, decoder, etc. from the A15, but that still doesn't explain the huge difference in pipeline depth between the two designs. There's such a large difference there, that even if the other elements are similar, that they're obviously not the same.
Also, there's nothing special about having a wider decoder. They're obviously going to have similar ancestry, but Qualcomm may have modified the design of theirs to some degree and stuck with their own design. Also, since all of the Cortex-AX chips work on the same instruction set, there's going to be a lot of similarity in the decoder anyways. The same goes for the FPU. Just because they implement the same instruction set doesn't mean that they have the same design.
What you're suggesting is basically the same as claiming that when Intel started making their 64-bit chips, they just took AMD's design and used that. Simply implementing the same instruction set does not mean that the internal design is the same.
Given that Qualcomm has been making their own customized cores since the Cortex-A8 days, it would take more evidence for me to buy into your statement. I'm less sure about Apple's A6, as I have no idea how long that was in development or even all of the available information about the chip to determine how related it is to the reference Cortex-A15 design, but you'd probably have a better argument there.
Simply having a similar feature set isn't conclusive proof.