google buildpack psycopg2-binary Error: pg_config executable not found
No, you are not wrong; I had the same issue today, and after a bit of googling I found this (solved) github issue: https://github.com/psycopg/psycopg2/issues/699#
The comments there (https://github.com/psycopg/psycopg2/issues/699#issuecomment-377188700) indicate one possible cause for the issue is using an outdated version of pip.
I was using version 18.x, and after upgrading it (python -m pip install -U pip, which got me pip 21.1.3), the installation of psycopg2-binary finished successfully (as expected) even though I don't have any postgres dev libraries installed.