Let's be a little more specific. "Reason Syntax" is merely a new syntax on OCaml, but ReasonML in general has a much larger scope - which includes every part of the toolchain/workflow up to and including package management, IDE integration, source formatting etc and more.
So Reason is to OCaml a little like Elixir is to Erlang - newer syntax, better tooling, same great foundation.
httpaf is quite low level. We need more abstractions on top of that for daily server development