I'm referring to their guiding principal that disclosure is necessary when that disclosure might change how a consumer evaluates the review. If it is reasonable to think the disclosure would make no difference, then no disclosure is required. Though I'll admit that can be interpreted broadly enough to say that disclosure is always necessary.
What is making you think this is true? It's not.
For example 3/4 of the example cases here are around undisclosed financial arrangements in influencer marketing, and in all cases the company admitted fault: https://mediakix.com/blog/ftc-influencer-marketing-violation...
I'm not sure what your definition of illegal is, but there is a law that the FTC is using to win legal cases on the issue.