Per 90.3, Chapters 1-4 apply generally to all electrical installations. That makes it mandatory. Chapters 5-8 do not generally apply, and only supplement or modify Chapters 1-4. Chapters 5-8 only apply when that application or equipment arise, so Chapters 5-8 are not mandatory for every situation, only those that they apply.
The statement was in context of the article, to reflect the idea of an optional Article 645.
The entire quote is "First, only chapters 1 through 4 of the NEC are mandatory. Secondly, chapters 5, 6 and 7 apply to special occupancies, special equipment or other special conditions. These chapters describe applications in which certain additional requirements are necessary or in which certain exceptions can be made to the requirements of the previous four chapters."
I think when taken in context of the entire text of the article, the statement is OK.
