I've never heard of unity builds including everything together. I'm not aware of a build system which even does that by default. If you enable unity in CMake, it groups batches of 5 files together by default, and the documentation discourages users from including everything in a single translation unit.

sqlite calls it an amalgamation build. Whole program building certainly exists. It doesn't explain where the quoted poster got the idea that headers were intended to save space rather than allow for independent compilation.