Just to clarify, it's not concurrency that's the issue but parallelism. You can write nice concurrent code pretty easily, but writing code that runs on multiple cores is still a problem. Also, if you're interested in using OCaml for web programming, there is some pretty cool stuff you might wanna check out [1] [2] [3]. That said, there's no great solution to the lack of macros :/
You're talking about data parallelism, the OP is talking about task parallelism. Since in both cases things are occurring in parallel it makes sense to use 'parallelism' as an umbrella term for both and it was clear from context here that it was task parallelism under discussion.
[1] https://github.com/dannywillems/ocaml-for-web-programming
[2] https://github.com/rizo/awesome-ocaml#web-development
[3] https://facebook.github.io/reason/ [2]