I think he is making the distinction between 3 different categories that readers are in general lumping into the 'interpreter':

1) Python is executed by an interpreter (necessary overhead)

2) Python as a language is so dynamic/flexible/erfonomic that it has to do things that have overhead (necessary complexity unless you change the language)

3) the specific implementation of the interpreter achieves 1 and 2 in ways that can be significantly slower than necessary

Seems he is pointing out that a lot of performance issues that are generally thought to be due to 1 and 2 are really 3