It seems that he was wrong.
According to my understanding, he posted here first that he used testing and was getting snapshot but later he posted in the M7 thread trying to install the milestone where the sources were pointing to bintray (so the openHABian config tool updated them correctly).
Snapshots are on openhab.jfrog.io and his apt doesn’t use it (of course, he could have manually edited the lists).
I found this: Openhab2 2.4.0-M2 - #3 by Benjy
I think that this is up-to-date: https://github.com/openhab/website/blob/master/.vuepress/public/v2.3/docs/installation/linux.html
edit: I just checked and the testing repo does not contain Snapshot builds. Only Stable and Milestones. So he couldn’t be receiving Snapshots from testing.