If you check the first part of your .pnts file (either with head or with your browser's dev tools) you can see something like: "pntsè,ì+X{"BATCH_ID":{"byteOffset":..." which indicates that each point has a BATCH ID. Now if you look at the Batch Table of your file, (which in the case of this file that was made with the point-cloud-tiler binary is at the very end) you will find this string: '{"Classification":{"byteOffset":0,"componentType":"UNSIGNED_BYTE","type":"SCALAR"}}'.
Unfortunately it does not seem like CesiumJS supports styleable properties that are defined via batches: https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/Scene/PointCloud.js#L458 so what is written above does not work for this type of files (which may or may not be your case).