Edema is related to soft tissue swelling. It is not a term used for muscle. Someone took the fact that your muscles can absorb water (which anyone who has ever carb loaded already knows) and tried to make that a negative. The sad thing is, there will be uninformed people who fall for it.
Just to make it clearer, it is like someone saying, "Don't lift weights because it causes severe 'myo-gigantism'". In other words, it causes muscles to grow because "myo" refers to muscle and "gigantism" refers to overgrowth. Since I used a word that sounds like a disease, however, you might actually think this is a bad thing.
"Intramuscular edema" is simply using random medical terms to say the muscles increase in size due to hoilding more water. Gee, that is what creatine does. But hey, it sounds like a disease.