Currently, it seems there is some bug going on with their build system or something. I actually had lodged an issue of my own (and even had submitted a PR to correct the documentation) before someone replied, pointing me to the root issue. So for the time being, instead of using https://dl.bintray.com/openhab/apt-repo2 location, you should use https://openhab.jfrog.io/openhab/openhab-linuxpkg repository location.
I ran into the same thing. Specifically, to get around this, the second instruction under the OpenHAB portion of the original post can be substituted with this:
echo 'deb https://openhab.jfrog.io/openhab/openhab-linuxpkg stable main' | sudo tee /etc/apt/sources.list.d/openhab2.list
EDIT: I've just noticed that they've updated the Package Repository Installation instructions to add the command above.
Further, if you care about issues such as software freedom (which I do a great deal) you might want to consider installing Azul/Zulu Java 8 (make sure it's 8!) which I have done, instead of Oracle Java. Azul's (also excellent) instructions can be found linked from the OpenHAB Linux install instructions. And that would also solve your missing repo problem, obviously.
If using Zulu Java, beware that the repos are not being maintained and are behind (see reference for manual instructions).
I used OpenJDK 8 for java with
sudo apt install openjdk-8-jdk-headless.
I'm using an Ethernet gateway (on a separate Raspberry Pi) though which is a twist on all of this.