RG is a liar. The motivations report says this about the excrement:
"A sample of feces and two pieces of toilet paper were taken from the toilet in the larger bathroom. The paper yielded the profile of Rudy Hermann Guede, both on total DNA testing and on testing for the Y chromosome alone. The feces gave no result either by DNA analysis or by Y chromosome analysis."
"Samples were taken from the toilet paper because this paper, used in wiping, is much likelier to contain epithelial cells, and they did indeed yield the genetic profile of Rudy Guede."
"Further biological traces of Rudy Guede were found on the toilet paper taken from the toilet of the larger bathroom. The faeces present in the toilet of that bathroom did not, however, yield any results, and Dr Stefanoni, the biologist of the Scientific Police, explained that the presence of numerous bacteria easily destroys what DNA might be found in faeces."
This could have very well been somebody else's excrement and RG just being helpful