Correct, buster is only in nightly channel. You can use
curl -fsSL get.docker.com | CHANNEL=nightly sh
to install Docker on Raspbian Buster 2019-07-10-raspbian*.img
until next stable release of Docker CE.
If you want to install a stable release now, you can use
curl -fsSL get.docker.com | sed 's/buster/stretch/' | sh
which installs 18.09.0 at the moment.