There is an excellent discussion of the Kashrus of gelatin here. Here is a summary of the section that relates to your question, and I have looked through most of the sources, and can confirm that they are accurate:
There is a discussion in Beis Yosef and Shulchan Aruch YD 99, as to whether or not in a case of Bittul, we will count dry bones (or the bone part minus the marrow part) of the non-kosher component as kosher or not. Shulchan Aruch paskens (based on Ran, Rashba, Rosh, Rash, etc. mentioned in Beis Yosef there), that the dry bones count as part of the kosher component to help be Mevatel the non-kosher component. Rema agrees in principle, but advises one to be Machmir if possible or if there is no major loss (he quotes a few opinions in Darchei Mosheh that would support such a view, such as Or Zarua, etc).
Deciding on this Halacha was (and still is, to some extent) a major source of controversy as it relates to gelatin, as gelatin can be made from non-kosher dry animal bones. Various Poskim and Kashrus organizations have expressed their views. The OU claims claims that the consensus in the United States was/is that such gelatin is considered "non-kosher". The current discussion relates to other gelatin sources and production methods.
The opinion of the Rambam (Maachalos Assuros 4:18) is unclear, based on what you wrote above. The main approaches outlined in the article are:
- Magid Mishna there - the Gemara (Chulin 117b) quotes a Beraisa that the bones (similar to the skin, horns and other dry animal parts with no flavor) are not "Mekabel Tumas Neveilah", meaning, the rule of Neveilah (and by extension, non-kosher animals, as specified in the Rambam there) only applies to the edible, meat parts of the animal, not to these parts (although according to that Beraisa they are still considered food, as they are "Mekabel Tumas Ochlin"). Thus, the Rambam is referring to dry bones, and still says that they are not allowed to be eaten, likely for some Derabanan reason. (Note that the OU, in the article quoted above, Paskens like this understanding.)
- Rabbi Chaim Ozer Grodzinski (Achiezer 3:33:5) understands the Rambam as referring to bones with some marrow traces, and as a result, understands that for completely dry bones, (even) the Rambam would Pasken that it is completely permitted to eat them (against the question's assumption). There is less of a question why the Rambam would write that they are prohibited according to his reasoning.
The article does not discuss (in detail) reasons for why dry bones would be Assur or Muttar according to the Rambam. Here are a few of the suggestions made by various mefarshim as I understand (see Frankel Sefer Hamafteach for sources):
- No flavor (not Nosein Taam), thus Assur Miderabanan
- Considered Assur, but not a full Shiur (this is not the time and place for a discussion of Chatzi Shiur...)
- Not considered edible/food at all (the question's original assumption)
- Not considered to be "meat" and thus does not become Assur as meat
In summary, it is clear that many Poskim say that even dry bones of a non-kosher animal, such as a pig, are not to be eaten. Even within the opinion of the Rambam there are a variety of understandings and final conclusions, as noted above. The final ruling seems to be that it should be avoided for serious Halachic reasons, although there have certainly been Poskim who have permitted it in the past.
(For further reading, with an emphasis on the 20th Century Teshuvos published on this topic, see here.