I have looked at the plugin source code. They use the bridgesupport files in ruby motion to build the class and method trees that are used for code completion and other things. Unfortunately, the bridge support files do not include any inheritance cross reference so I think... and I am only guessing... they are using the Cocoa Documentation to find these references and the Documentation structure I believe was changed in either Xcode 7 or 8.
Even though JetBrains claims that the RubyMotion plugins are "open source" I have discovered that some files are missing as well as many library dependencies.
What might make more sense now is to maybe look into to creating a add-on for Visual Studio Code or Atom. Visual Studio Code is growing enormously in popularity and may be a better pick for the future.