I am glad that the word "myth" is starting to appear on Bucks. There are plenty of them, and what you have been told is another one for my list!
The 12+/13+ is only one paper, as you know, and therefore will include only some of the 21 question types. The only problem is that you don't know which ones!
As the test is five questions longer (and five minutes shorter) there will be 12 of the standard 21 types, plus (possibly) an extra question type, an example of which is one my 12+/13+ Sticky.
Your child needs to revise/practice all 22 types.
My only question now is who this "Someone" is who keeps telling everyone all the myths! If I could only trap him or her ...