Once again, I think you are missing the point. Apple sells computers, which you could build yourself, and thus they occur in the same market. Thus they are competing, no matter whether Apple markets simplicity, because they sell computers. While yes, I find that it is highly unlikely that someone looking for a mac laptop is suddenly going to be swayed by building their own computer. However we are talking about a $5000 workstation, and I think anyone who is looking at getting one, unless they are a mac fanboy or have way too much money, would also at least be considering building their own.
Edit: Not trying to pick a fight here, btw, just trying to point out that building your own is valid competition for this kind of computer.