Correct, that core is for both. You do however need to tell Launchbox it is for both, Launchbox will never attempt to guess the platform or emulator to use for it, that is defined by the user. In your case you are using a core that supports the CD platform, but you have not told Launchbox to use that core for that platform. This is the reason for the associated platform tab in the first place, so that you can use the same emulator, or in this case the same emulator core for different platforms.
While Launchbox does try to automate as much as possible some user input is still needed, it just cant be expected to know of every emulator ever made and for which platform it supports, thats just not possible. So the way it works is you add any emulator you want to use, but..... you need to tell Launchbox to use it for that specific platform.
Also bear in mind you can use any name you like for a platform, so if you decided to call Nintendo 64 "ninty 64" for example Launchbox would have no idea what that platform is, or what to do with it, hence you "associate" a platform to that system and emulator.