This library was developed for use with Unity's Burst Compiler. At the time of writing, there were no libraries for many basic functionalities, such as lists or dictionaries without built-in references that would make them impossible to nest, as per Burst's limitations on C#. However, they've been since reimplemented by Unity. I'd suggest using their versions as they're actively supported. Look for them in the Unity.Collections package, in the LowLevel namespace.