Marco (author) is probably asleep at this point and could give a deeper perspective. He sort of hits on this when talking about disk latency... Depending on your setup and well just from some personal experience I know it's not crazy for Postgres queries to go at 1ms per query. From there you can start to do some math on how many cores, how many queries per second, etc.
Single node Postgres (with a beefy machine) can definitely manage in the 100k transactions per second. When you're pushing the high 100k into millions read replicas is a common approach.
When we're talking transactions, question of is it simply basic queries, bigger aggregations, and is it writes or reads. Writes if you can manage to do any form of multi-line insert or batching with copy you can push basic Postgres really far... From some benchmarks Citus as mentioned can hit millions of records per second safely with those approaches, and even without Citus can get pretty high write throughput.