We don't know how old Bioko Island actually is, or when the ancestors of humans might have rafted there.
The oldest known rocks on the surface of Bioko are about 1 million years old (see Figures 1 and 3 from Fitton). But rocks at deeper levels are certainly older (see the cross section of a typical volcano, with some possible age numbers added). Bioko may be as old as its volcanic neighbors Oku (at least 23 Myr old) and Principe (at least 30 Myr old). (See the paper by J. G. Fitton, 1987).
Viruses, parasites, and their DNA vary from place to place. We will eventually be able to use the absence of 'African' DNA in living humans to know if and when human ancestors were not in Africa. (Negative evidence.) We will also be able to use DNA in living humans to determine where human ancestors actually were during different times (Positive evidence.) An isolated island like Bioko (during times with no land bridge) would have endemic parasites and viruses that might be identifiable in modern humans' DNA.