I generally agree on your definition, but this is about sales, and so we can't go around and count how many people did or are going to put a programming language on their tablet or phone. (And how was anybody to know if I was going to put a programming language on that heap of parts I got from NewEgg last week?) I'd consider whether it was reasonably feasible for a programmer to put a programming language on a computer, rather than what it is when it ships.
As somebody who used to pay a good deal of money for compilers for his personal computers, I'm going to argue that having to spend a few hundred dollars to program on something doesn't make it not a PC. Therefore, I consider the iPad to be as much a PC as my Nexus 7, because the capability is there.
And Apple does not forbid iDevices from doing things. Apple forbids certain things from the App Store. If you jailbreak your iDevice or get a developer's license (and a Mac if you don't already have one), you can do lots of things on it that Apple didn't intend.