CoreLibs is a collection of Lua code built on top of the base SDK, convenience code we've developed over the years that didn't belong in the system core--though sometimes we do move code from CoreLibs to a native implementation to improve performance. A lot of the sprite code used to live in CoreLibs and now is implemented inside the Playdate runtime in C. The CoreLibs source is available in the SDK because it's useful to be able to read it and customize it, but also because that's just how Lua works. I guess we could have come up with a compiled bytecode library format and distributed it in that format instead, but that sounds like a lot of work and it wouldn't be as useful.
The base SDK (that is, all the functions listed in Inside Playdate that don't say they're in CoreLibs) is closed source, though I'm always happy to answer questions about implementation details (e.g. acceleratedChange in C SDK - #4 by dave).