It goes through the bitbake parser first, and by the time it is
executed, it becomes 'export ABC="AAAAAAAAAAAAAAAHHHHHHHHHHHHHHH"' - the
original ${ABC} is nowhere to be found, because it's a bitbake variable
(just like SRC_URI, for example), not an env var, it was substituted
with the literal value of ${ABC}. And before executing that export
statement, the ABC env var doesn't exist in the build environment at all.

Man, considering the quantity of search results telling users to use BB_ENV_PASSTHROUGH_ADDITIONS to accomplish what I'm doing, that really feels like something that should be highlighted in red, and bolded, in the documentation. Not to mention all of the CMAKE gotchas I've come across.