Are the app store review guidelines under NDA? To access the app store review guidelines you need to sign in:
https://developer.apple.com/appstore/resources/approval/guidelines.html

Does that mean they are under NDA or it it ok to blog about a specific requirement?
There have been several cases of people blogging about review guidelines, and AFAIK, nowhere does Apple ask us to agree to an NDA about the guidelines when creating our developer account (at least, they didn't for me).

Here's a few cases of the guidelines being blogged about:

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