No coordinate system specialist either but I believe LL84 and 4326 are the same coordinate systems. If you search for 4326 in the Coordinate System Gallery you will get 3 results, which have different names and descriptions but are the same system.
The EPSG codes are here to help GIS software to function, where a code is less interpretable than a name / description. When checking the Esri-JSON
documentation:
The value of the spatialReference key is a JSON object containing coordinate system data for the features in the data set. In ESRIJSON, coordinate systems are represented by their well known ID, typically an EPSG number. The spatialReference key and value are duplicated in the geometry object of each feature in the dataset. A spatial reference object representing the EPSG:4326 coordinate system is shown below.
{ “wkid“ : 4326 }
The value of the features key is an array of JSON objects, representing the individual features in the data set. Each feature object has two keys, geometry and attributes. The value of the attributes key is a JSON object whose keys and values represent feature attribute names and values. The value of the geometry key is an Esri-JSON geometry object. The various types of geometry objects are listed below.
So when FME reads LL84, it writes 4326. I think this is a technical design choice.
I can also imagine situations where you want to have multiple geometry columns per feature, having different coordinate systems. So I would prefer extracting and replacing the coordinate system when using the GeometryExtracter / GeometryReplacer.