What you have to remember is that for many teams, when their product takes off, they are not equipped with the deep internal knowledge of how to scale a particular part of their stack. This was an awesome story from a small team having to tackle those challenges, and how they were learning as they went. So, while there are some of those "can't you just", and "whats interesting about this?" comments here, with the narrative of the growth rate, and the very high profile of the product, it was the perfect user talk for a internal development focused conference.
The key insight, and main message, of the talk was that if you are not too write heavy you can scale Postgres to very high read throughput with read replicas and only a single master! That is exactly the message that needs to be spelled out as that covers the vast majority of apps.
As an observation, in the Q&A at the end of the talk the questions, primarily from core Postgres developers, were focused on learning about the use case, and not an opportunity to suggest that they were doing anything wrong (not quite the same as this thread could get). A genuinely awesome group of very friendly and welcoming people in the Postgres community.