Yea that's my assumption. Don't know what it takes to pull out seating, overheard, flooring?, etc. and what sort of effort that takes time wise. I assume it's not simple and takes time.
(am not an air cargo freight expert if you haden't realised already, so be gentle)
Obviously, you won't want to do this for general cargo transport, but for "hey, we have a grounded plane, but transporting air mail gives great rates right now" situations?
Limited loads could be secured in the passenger cabin using the same tracks. Whether or not that actually makes economic sense is a different discussion.
You can see this in action here: https://www.reddit.com/r/aviation/comments/8scjkk/ever_wonde...