Even though the Mechaber also wrote that it may be cooked afterwards, nevertheless the Rema wants to teach us that it is like all other meat and can be cooked with other meat as well.
 The reason is because we fear that one may cook it without having cut it, but roasting is permitted by cutting it open and slightly salting it like all other meat. Nevertheless, post facto, if one cooked it after cutting it and salting it then it is undoubtedly permitted even according to those who are strict. It seems that one should not eat the heart of an animal since it causes one to forget his learning. One should be careful even regarding the hearts of poultry.