How is yellow elastic cartilage a true cartilage?
True cartilage is something of a simplification or a misnomer, they are saying that it truly a form of cartilage and not another elastic tissue type. in fact 
"true cartilage" is a term coined by your book. They could have also said it's part of the cartilage family of cell/tissue types. 

It's similar to cartilage under a microscope, structurally and has similar consituents, although yellow cartilage also contains elastin. If you were to dissect humans, you would probably find that there are a lot of variations of cartilage of different elasticity, midway in between both forms. 

this page is full of info and pics:

It's a good idea to check wikipedia and 2-3 pages to find results to that kind of query, you will find fabulous resources, whereas on stack it is really for studies that take more than 20 minutes via google.