... because functions in pure FP never have side-effects, their reduction order can be entirely undefined. Actions defined by those pure functions, however, are defined as always sequential when main is executed.

Yes. In one sentence you have summed up the requirement for defined EO in languages, or those parts of a langauge, that have side-effects. Where it gets awkward, is to see how that enables the parallelisation of functions that can have side-effects, when they are a part of the same expression.