because the thing that needs to be maintained is the causal ordering of events. For the example from TFA, block -> tweet and tweet -> block have the causal effect of the blocked user seeing or not seeing the tweet respectively. Phrasing it as causal ordering removes time from the equation, which you aren't always able to take into account with distributed systems. Causal ordering also allows for causally unrelated events to appear in any order, for example updating your bio and posting a tweet. Check out the wiki article on logical clocks for a rough starting point in delving into this topic: https://en.wikipedia.org/wiki/Logical_clock.