In Facebook Messenger, regarding the question: How does the messenger maintain the sequencing of the messages?

Is the method described analogous to consistent prefix read? see https://www.microsoft.com/en-us/research/publication/replicated-data-consistency-explained-through-baseball/?from=http%3A%2F%2Fresearch.microsoft.com%2Fpubs%2F157411%2Fconsistencyandbaseballreport.pdf