The phrasing seems weird because of how knowledge is acquired in this area. The first achievement was discovering that indeed some blood group antigen exists,

The phrasing seems weird because of how knowledge is acquired in this area. The first achievement was discovering that indeed some blood group antigen exists, and the second is figuring out what that antigen is. As an analogy, a bug can be discovered (e.g., UI glitch), but what the bug in the code “is” may remain unknown until a deep dive into the code, and fixing this bug can only happen once the cause is known. Each of those steps can be entirely separate endeavours.