I think the article is reasonable but it suffers from the imprecision of "senior" as many things do. What is a 'senior' engineer in a startup, or at a mid-size private company, or a large public company? All of those companies have "senior engineers" but they aren't easily swapped out for each other. These are my definitions, at a startup a senior engineer is one that is broad enough in their experience to have gone…
Why should not all those be together and be the bar for senior starting at the start-up layer? I think the only big difference is working/navigating in a small or large org, no?
At a start up, even if you have your first customers, you want to deliver a product or more features as quickly as possible starting from scratch. Possibly, you also want to be able to pivot totally if the first idea does not stick.
It is great if you can do well in both environment, but usually, this is different skill set, and I would even say personal inclination. You'll likely have a hard time to convince a start up engineer to work on COBOL code base on a mainframe.