Possibly when Sibley mentioned two different species he was claiming something like Elaphe guttata X Pituophis catenifer affinis. Which are two different species even if they dont belong to the same Genus... Or unless Elaphe emoryi and E. guttata are no longer a species of their own ( I know the whole Genus changed to something else but cant remember the name and whatknot) then the Elaphe guttata X Elaphe emoryi are two species and a hybrid. Surrporting Sibley's statement on "Hybrids are the offspring of parents of two different species."
If E. emoryi and E. guttata are no longer their own species, even in the new Genus, this surrports the "since noone tends to agree on what constitutes a species ( Species change, ssp. change ) Making a former hybrid possibly a hybrid no longer, or into a different hybrid mixture." statement.
Either way, people have their things. I prefer to keep mine "Pure"