I'm sorry for the confusion. I'll try to update the documentation somewhere..
The CkString and CkByteData classes won't be available in .NET Core (it's intentional).
You should instead use the Chilkat.StringBuilder and Chilkat.BinData classes.
The CkString and CkByteData classes were original classes created about 20 years ago, and don't fit well into the multi-language API design. They were created with C/C++ in mind. Obviously they'll remain present in the places where they currently exist, but for any new languages supported going forward, they won't be included. If there's something CkString provides that Chilkat.StringBuilder doesn't, then let me know..