How do they define the term 'portable computer' anyway?
If you accept that "A computer is a programmable machine that receives input, stores and manipulates data, and provides output in a useful format." and accept portable is something that is "able to be easily carried or moved" then the iPad is a portable computer by definition.
However, the problem isn't really a question of if the iPad is a computer or not, but more a question of how to group the computers they compare.