The issues discussed in that link, such as concurrency, lack of atomicity and failure are all factors that make bi-directional sync a hard problem. Yes POSIX provides few guarantees and so that burden rests on the application code above. The formal model must consider the race conditions and failure modes, it is not "the easy part". I do acknowledge there is a modelling gap to overcome and some problems with POSIX, but I disagree that its the hardest part of the problem.