I was deliberately careful with the wording - yes, in distributed systems, we may be unable to determine which event came first. But one of them did happen firs

I was deliberately careful with the wording - yes, in distributed systems, we may be unable to determine which event came first. But one of them did happen first in reality, we just can't tell which. However, in some cases (see the Wikipedia link) it's not even true that one of them did happen first - it depends on where you observe them from and there is no definitive answer! Relativity is weird.