Strange, this does seem to work with python3.8 on ubuntu 20.04 (the site-packages shows up in sys.path), but for me in a virtualenv bin/python is a symlink to the system python, so how does python 'know' what path to use? Is there logic baked into the interpreter?

I seem to recall that with python2.7 that calling bin/python in a virtualenv without activating the virtualenv did not used to "work" (i.e. it would use the system packages). Did this change at some point or is my memory just wrong?

Yes, if the python interpreter the poetry environment utilizes is in your $PATH.