The para Amino Phenol developer used in Rodinal is possibly a weak color developer and the E6 film has a coupler excess. You may be getting a tiny amount of dye formed. That, combined with retained silver, may give you enough density to image with.
Just a guess.
* I am editing this with afterthoughs about how powerful pAP may be in coupling at the pH of Rodinal. I just don't know, but it does have a free amino group that could couple at the high pH and given enough coupler.