They describe another real weakness in the protocol on page 4 that also makes the single relay indispensable: fedi has backfilling problems, but its possible to solve them because you can at least know who does have the complete picture - the OP server knows of all interactions (that it wants to). Since there are no backlinks, and PDSes are not dereferenceable by username, the only way the whole thing works is if someone has a relatively complete picture of the whole network - otherwise eg. you would have no idea who to deliver a post to.