You are correct. Tieflings can be of any race. They were always generalised to be humans because humans are the most common race. There are other labels for other races that has interbred with fiends - fey'ri and quicklings for example -- but I am unsure how much these resemble tieflings (in that tieflings are completely mortal with some fiendish influence) or if they count as actual fiendish beings.
The 4th ed tieflings however are all descended from humans. Though that origin is far ago, and they are now a true breeding race -- they are no longer born from humans and will not have humans children themselves, only tieflings.
