There is quite a lot of intersection, for many reasons.
- Haiku does not force people to work exclusively for Haiku, developers are free to hack on whatever they want (with maybe the exception of developers being paid to do something, and even then, it can be discussed)
- People working on ports often end up uncovering bugs in Haiku and maybe fixing those and becoming Haiku contributors
- All of us (or at least most of us) are first and foremost Haiku users, and sometimes, porting existing software is indeed the best or fastest way to get something done.