I don't think his point is so much about the source as it is about updating N containers. For instance, say there's a known libssl bug. Can you tell how many of your containers are running that version of libssl? And how do they get updated?
1) List the number of containers running pre-fix versions of images of libssl-using server software. 2) Bump the version of the images you're using as a base for your server images to post-libssl-fix and push.