Some 4.20 development statistics - core kernel distinction

These don't mean the same thing, and in fact don't make much sense without additional context. As you mentioned, 'kernel' has a specific meaning in computing, while 'core' is used more generically[1].

The phrase 'core-kernel' is short for 'core of the kernel', i.e. it implies possession: the kernel has a core. We already know what 'kernel' refers to (the Linux kernel), so it makes sense to refer to its core without further explanation. Applying the same reasoning to 'kernel-core'---which expands to 'kernel of the core'---the phrase becomes confusing, because 'core' is a less specific term than 'kernel' and it's not clear which core we are referring to. 'Core' can't be referring to the Linux Kernel (because otherwise we just would have said 'kernel' rather than 'core'), and it's not obvious what else it might refer to, since it's such a generic term.

Side note: If the phrase did not have a hyphen (i.e. "core kernel developers"), I would have parsed it as "core (kernel developers)" rather than "(core kernel) developers". Meaning 'those developers who are at the core of the kernel development community'.

[1] Except when it also has a specific meaning, e.g. in 'core dump'