You are correct where if one is not capable of abstract reasoning, then he is not capable of enlightenment. But being capable of abstract reasoning alone, doesn’t ensure that he is capable of enlightening.
The deciding factor given in Abhidhamma is the “Type of birth” that ensure sufficient wisdom and merit.
There are 3 types of births called:
- Tihetuka Patisandhi (Thee-roots Birth)
- Dvihetuka Patisandhi (Two-roots Birth)
- Ahetuka Patisandhi (Root-less Birth)
Tihetuka Patisandhi is the highest form of rebirth in sensuous plane. Only the beings who are born with a Tihetuka Patisandhi are capable of enlightening in this very life.
Gods and some human beings have got a Tihetuka Patisandhi while all the animals and some human beings have not got a Tihetuka Patisandhi.
We find many people in Buddhist countries who practice Dhamma and still having doubts about whether there are Tihetuka or not.
If you like Abhidhamma, you can start reading the road map Abhidhammatthasangaha for clarifying such issues easily.