I don't think it's a prequel - he just wrote it before TBP. There is one character in common with The Dark Forest, but only to the extent that this character is regarded as a great physicist in both books and makes some discoveries.

I've read (and thoroughly enjoyed) them all, didn't even know Ball Lightning was a prequel (it was written after the trilogy). Perhaps some repeat characters?

I don't believe there are any spoilers either way.

I only know about it being sort-of a prequel because of the afterward material in the audible audio book. The books can be enjoyed in either reading order.