Interesting... So they are using ObjFW as their ObjC runtime, which is relatively an old implementation. So if your app has some Obj C Runtime use, you may see trouble here. It would be interesting to test their other implementations.
The problem is not the syntax support. The ObjC runtime had many changes to it done internally over obscure flags to support Swift and other compiler features. These are not readily documented on Apple open sources. (Perhaps more documentation exists in LLVM.) Once the runtime contains ABI changes, it makes it difficult to port apps that (ab)use the runtime. Not a problem for some apps, but a big problem for others.