> working at a well run startup

True, but this also assumes that you can 1. identify from the outside (and as a very junior engineer!) which startups are well run and 2. actually get into said well run startup.

There are plenty of poorly run startups as well, where the things a young engineer would learn could actively hurt their career later on. Since junior engineers are by definition not very experienced, they cannot yet tell which startups are good and which aren't.