The guarantee is based based upon the thought that water runs down hill. Or in this case that the poop will successfully make it to the treatment plant. Just as an FYI, whole turds never make it to the plant. They disenigrate by tumbling through the pipes and liquid. Visit your local plant and ask to see the stuff they screen out. Also ask them about the money they find.
“Visit your local plant and ask to see the stuff they screen out.”
Ten minutes prior to seeing your post, I told my son about my trip to a sewage treatment class in college.