The gemara in Sanhedrin 69b explains that the machlokes Bais Hillel and
Bais Shammai whether or not an 8 year old katan's act of biyah can posul his mother is dependent upon whether or not we learn out from doros harishonim to doros ha'acharonim or not. The gemara proceeds to try to locate a case where we can show that in doros harishonim an 8 year old katan was able to father a child.
Why are the two dependent upon one another? As the gemara explains earlier, it is very possible that the biyah of a katan is considered a biyah because he has zera, and yet his zera is not developed enough to be able to create a child. In order to support Bais Shammai's opinion we should not need to find a case where an 8 year old child fathered a child but rather it should be sufficient to show a halacha which indicates that his biyah is considered a biyah.
Let me answer your question with another question. The CHIDUSHEI HA'RAN asks how Beis Shamai can learn the age of physical maturity from Doros ha'Rishonim, if we see that today people are different and they cannot have children at that age! The Ran answers that the Beis Shamai is making exactly the distinction that you suggested. Although men cannot have children today at the age of eight, we can learn from the Doros ha'Rishonim that even today, a child is considered *Bi'aso Bi'ah* at the age of eight.
This answers your question as well, since we are not trying to prove that an eight year old can have *children* at the age of eight today, but that if Doros ha'Rishonim could have children at that age, our generation is considered Bi'aso Bi'ah at that age.
Beis Hillel argues and rules that since today an eight-year-old's Zera is not yet mature, we should not consider him to be Bi'aso Bi'ah either.