> in Erlang, preemption happens at function calls

I didn't think this was true. The secret sauce was that Erlang preempted on reduction counts, so as a result nothing has any responsibility to yield itself.

edit: ah, literally no difference between the two in the implementation.

That's mostly an implementation detail though, the thread switch could happen just about anywhere. Function calls are a convenient stopping point but the way the reductions are implemented in Erlang it could theoretically be done on any boundary.

It's an implementation detail, but it's very convenient and simplifying, like a lot of Erlang implementation details.

If you take out enough simplifying implementation details, you no longer have a powerful system built from simple parts.

In this particular case though I don't think it would matter much. I've looked pretty closely at how the BEAM does it's thing and it's a work of art, this particular trade-off is more one of convenience than of necessity. But you are right that Erlang is full of such clever (and beautiful) tricks which make it a very interesting codebase to study.