Yes -- but it has not always been so. No explicit rules were given in the AOU Code prior to the 1908 edition, but Richmond in 1902 was probably using unwritten rules similar to those that were added at that time. See: https://babel.hathitrust.org/cgi/pt?id=hvd.32044062470786&view=1up&seq=64Art. 56.2. One-letter difference
Even if the difference between two genus-group names is only one letter, they are not homonyms.