I was also a bit confused by that part. What is it about Postgres that prevents rapid prototyping? It's not like the schema is set in stone, or even particularly hard to change once you've created it.
Well, yes and no. If you already have your database running in production with many transactions and live connections 24/7, changes to the schema might not be hard per se but always needs careful planning and execution. Additionally, in Postgres a change like adding a column locks the whole table (although I hear this is going to change) so writes are off for a short time. If this becomes unacceptable, you go for a blue/green deployment which has its own gotchas.
So, while I agree with your main point - there is nothing in Postgres that prevents rapid prototyping, and I would chose it over Mongo any moment, I understand why some people might prefer the more "dirty" approach.
True enough, and the solutions for lighter-weight schema changes have evolved and weren't always so good, but early stage startups often don't really have the sort of data-weight issues that make this hard, unless they're starting with large data sets already. Even at the scale of small hundreds of millions of rows per table (like the company I'm in charge of the database for) it's not much of an issue.
I had to look it up, apparently adding a column with a non-null default was "fixed" in PG 11 (2018), but with a null default it had been fast for a while:
