It probably does have different modes that it selects based on the input data. I don't know that much about the implementation of image compression, but I know that PNG for example has several preprocessing modes that can be selected based on the image contents, which transform the data before entropy encoding for better results.
The difference with OpenZL IIUC seems to be that it has some language that can flexibly describe a family of transformations, which can be serialized and included with the compressed data for the decoder to use. So instead of choosing between a fixed set of transformations built into the decoder ahead of time, as in PNG, you can apply arbitrary transformations (as long as they can be represented in their format).
