Yeh - the problem with this assertion is that there is conflicting evidence regarding it. As stated, chimps and gorillas have type A and type B blood respectively, with minimal O in each population.
This introduces an evolutionary difficulty - EITHER both chimps and gorillas independently mutated the same blood types as humans, OR A & B blood types existed in the common ancestor of humans, chimps, and gorillas.
Secondly you are ignoring the evidence that certain S. American tribes are 100% type O - but this is not found in Africa at all.
Thirdly the Ainu are 18% AB -- and they have been isolated in Japan for many thousands of years. The amount of AB increases (in general) the closer geographically populations get to the Ainu (see my other posts and links).
Therefore -- the assertion that type O was the "original" type is an old hypothesis, and repeated in many low level textbooks. But it is NOT supported by modern data. Hey, that's the good thing about science -- it moves on. When it gets new data, old hypotheses get revised.