My book has only mentioned the fact but no reason is given so I would like to know the reason for it. I would like to know what more to add in the body but the question just doesn't get posted.
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.
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.