It doesnt scale vertically either. Postgres is single-threaded meaning it can't make use of multiple CPU cores on the same machine. There have been some slow improvements to this and 9.6 seems to hint at some parallel aggregation changes but overall Postgres is strong in features but weak in scaling (in any direction).
It looks good, will have to wait and see what actually makes it into 9.6 and how well it supports all the possible queries. This is something that many commercial databases have solved so Postgres is pretty behind in vertical scaling.
British English vs. American English? I dunno if the poster is British but I am. I read that they haven't done enough to call pg vertically scalable but recent work suggestions (hints) that they are working toward it. The expression is more metaphorical than literal, an Atlantic divide my American colleagues explicitly warn me about when I am writing my perf reviews.