My time to shine! I've spent yesterday morning to track the photo down and answer this question. The APOD description is lacking. Yes, this was an exaggerated stack of 153 four-second exposures (the rejection map of the satellite trails was added on top of the image), and the gaps happened when the camera took its time to save between two exposures.

Here is a link to the original photo and it's description (German) by Uli Fehr: https://www.facebook.com/groups/Nachtfotografie/posts/264063...