This is indeed a very interesting question to ask. In order to help you answer this question, I would first of all suggest to focus on the fact that not all students are the same. Each student will have a different background, and each student will have different knowledge levels and skills. Therefore, what might seem basic to some students, might not seem basic at all to somebody else in the same class. It is very dangerous to expect all students to have the same level of ability, given how different students really are.
You might want to point out that it is important to indeed start with the basics when teaching. For example, when teaching to read and write, a teacher would naturally focus on simple language first, to ensure that the students are able to understand the principles behind reading and writing, before challenging themselves further. Therefore, initial teaching of young learners will undoubtedly be fairly basic. However, teaching usually involves a degree of differentiation, which means that teachers aim to teach their students according to their individual ability.
As I explained above, the same task might be easy for some students but difficult for others. Therefore, a good teacher will support the students who are in need of more help by focusing on more basic and simple language or topics. At the same time, a good teacher would then try to stretch the more able students by providing them with extra activities, which will be more advanced in terms of topics or language. It would be wrong to hold more able students back, just to keep everybody working on the same level. Differentiated activities, which target a variety of ability levels, will allow students to develop their skills and knowledge at their own pace.
With this explanation, it is logical to conclude that the statement in the above question is a myth.