Why isn't Iberia a tectonic plate?
The Iberian plate may have been a sub-plate once-upon-a-time, but in modern maps it is usually shown as part of the Eurasian plate, and the next plate to it is the African plate. The same thing applies to the so-called Indian plate. Prior to 43 million years ago it was indeed a separate plate, but then it fused with the Australian plate and there is no longer a plate boundary between India and Australia. The resulting plate is therefore usually referred to as the Indo-Australian plate. 

A lot can happen in tens of millions of years, and one of the things that can happen is a change in plate boundaries. There is no longer a plate boundary between what was the Iberian plate and the Eurasian plate. If you are referring to events which happened tens of millions of years ago, it might make sense to refer to them a separate plates, which is why the article you quote refers to an Iberian sub-plate.