If it was just about NoSQL vs RDBMS they could have used Postgres as a key-value store instead of mySQL. So yes they absolutely did endorse mySQL over Postgres.
Also, what is the scale of Uber that you think is unsuitable for RDBMS? Uber does a million rides per day but these rides are very predictably local. I don't know enough to make any firm claims here but at first sight this doesn't look like the sort of scale that is unachievable on an RDBMS in principle.
They endorsed MySQL over PostgreSQL for a type of structure and access very few people use. I don't see how the query language plays into the recommendation at all.
Well, Uber claims they are using MySQL because it's a better NoSQL database than Postgres.
From the context, I don't think they ever evaluated Postgres NoSQL features. But I don't think they would get a different conclusion if they did. MySQL trades some consistency guarantees for speed, and it looks like Uber doesn't need those extra guarantees anyway.
"Also, what is the scale of Uber that you think is unsuitable for RDBMS"
Never said it's unsuitable, I have no idea. Just said the move from relational to NoSQL is what most billion dollar companies with mass-market apps or websites do.
Also, what is the scale of Uber that you think is unsuitable for RDBMS? Uber does a million rides per day but these rides are very predictably local. I don't know enough to make any firm claims here but at first sight this doesn't look like the sort of scale that is unachievable on an RDBMS in principle.