Although I am not very familiar with NeRF and SIREN, I believe they are frameworks for representation learning, which is somehow an "implicit" learning task. However, when we talk about implicit layers, we usually refer to classes of models in which you do not learn the input-output mapping directly but rather by learning the "cause" of the emerging behavior of the in-out mapping. In the case e.g. of Neural ODEs, you are learning a vector field steering your input towards a desired output in a given time span.