Oliver Cowdery initially wrote Shemlon in 𝓟, but then almost immediately he corrected Shemlon to Shilom (there is no change in the level of ink flow). These two place-names, Shemlon and Shilom, are visually and phonetically similar, so the mix-up is not surprising. Moreover, in nearby passages these two place-names are frequently found in close proximity to each other, which adds to the possibility of confusion:
Note, in particular, that the Shemlon in the second of these passages immediately precedes Mosiah 11:13 and is most likely responsible for the momentary mix-up in 𝓟. In fact, in that manuscript the name Shemlon in verse 12 is located two lines right above where Oliver initially wrote Shemlon in verse 13. Further, verse 12 explains that the land of Shemlon was possessed by the Lamanites (see Mosiah 7:21 as well as Mosiah 9:6, 8, 14 for evidence that the people of Zeniff inhabited the land of Shilom); clearly, king Noah would not have had any buildings built in the land of Shemlon.
Summary: Accept in Mosiah 11:13 Oliver Cowdery’s virtually immediate correction from Shemlon to Shilom; based on other passages, Shilom is definitely the correct place-name for this passage.