FWIW I asked Chris about the author's suggestion and he indicated that he's never had a case where he found it necessary to create multiple distribution DBs to alleviate performance issues.
Great discussion!
For what it's worth, I have seen scenarios where multiple Distribution databases provide a performance improvement.
For example, let’s say we have two Publications that form part of an application platform. One Publication is sourced from a very intensive OLTP database and another Publication has relatively moderate activity. Given that both Publications share the same Distributor database, the OLTP Publication could be responsible for 80% of the overall activity to the Distributor, thereby adversely affecting/constraining the performance of any other Publications that share the same Distributor.
By providing a resource intensive Publication its own dedicated Distributor database, you can isolate the performance from other Publications/Applications.
Who knew Replication could be such fun 🙂