Ah yes, looking at the source code makes it a bit obscure, because the inheritance patterns are described in the wrappers. But for generators, you can do, for example osc.append(buffer, channelCount);, because generators are by nature multi-channel, but for effects you have to create a multi-channel instance, like flt = audioLib.LP12Filter.createBufferBased(channelCount, sampleRate, cutoff); and then you can use the append function similarily to generators.

This is a design choice to make implementing new effects simple, and the framework providing this extra functionality for you.