So even though Apple uses x86 processors and technically it is a personal computer, "PC" still refers to the old brand name that is synonymous with Wintel. Apple has personal computers, Microsoft has Personal Computers.
Of course it is, and much of what is sold with that moniker today is really more of an entertainment appliance than "computer." When my daughter was 5 or 6 and being told to "dial 9-1-1 for emergencies" she asked me "Why do they call it dialing?"
So the interesting thing from the perspective of the Steam box is this is an entertainment console with a bunch of details about how the insides work. I'm sure that 95% of the people who buy it will have the exact same experience they would have had if they bought and Xbox One or PS4, and the other 5% will have a lot of fun making it do amazing things and not have to worry about getting sued or arrested for it [1].
[1] http://en.wikipedia.org/wiki/Sony_Computer_Entertainment_Ame...
When the term PC was coined, computers were fridge sized horrifically expensive things that ran in research institutions and very large companies. The idea of having your own was...crazy.
It is weird how the name has come to mean something based on the Wintel nexus, but that's marketing and differentiation for you.
Those sound an awful lot like PCs to me. Sure, they have a bit of custom hardware design, but for the most part are basically just PCs; except that they're locked down and you can't write your own software for them without going through an intermediary (and if you try to, you may get sued or arrested for hacking).