I tested the current version and it made the new build properly available for Internal Testers, but not for External Testers. Note, I did not start a new version of the app, just pushed a new build (with incremented build number, but same version number), not a new version.
This setup, that the step does not make the app available for External Testers actually makes sense, although might be a limitation / possibility for a future update. External Testing of the build requires a review from Apple, and the build can’t be propagated to External Testers before that review. The review (AFAIK) also requires a couple of mandatory information, e.g. change log, which this step does not require.
Maybe we should note this better for this step, but right now the purpose of this step is to automate the uploading of your builds and the propagation to internal testers. Ideally, once the build is available, your internal testers (QA team) will pick up the build and test it, and you only release it for external testers once the internal test was successful.
Of course, if you don’t have an internal testing team, this might be a limitation, depends on your development workflow.
In any case, thanks for the infos @AndyDunn!