PerlinNoise is described as a method, but is actually a class that can be used to generate noise with the Sample method (FlaxEngine/Source/Engine/Utilities/PerlinNoise.cs at master · FlaxEngine/FlaxEngine · GitHub). It also shows as returning a value 0 to 1, but a sample is actually -1 to 1 multiplied by amount and offset by baseValue.
The current page appears to reflect an implementation of the cpp noise in C#, but that is not what currently exists.
Just the documentation was confusing until I looked at the source.