Is it possible there's some wireless communication going on via electric fields that we're not yet aware of?
Quick google search suggests this is a thing: https://www.sciencedaily.com/releases/2011/02/110202132617.h...
Could it be happening on a large enough scale to allow communication between hemispheres though? I'm not suggesting this would take over for the corpus callosum, but rather that it may be happening for everyone, so the true amount of transfer even after the corpus callosotomy is higher than expected?