Depends on what your definition of senior engineer is... For me it means you've spent a lot of time coding and mastered a few programming languages. You're able to make use of all that they have to offer when appropriate. You understand more advanced concepts like code readability and reflexively think about long term maintenance of the code you're writing. Starting to think about larger systems, maybe also branching out into team lead type of management responsibilities, or deepening technical skill sets related to infrastructure or particular areas of expertise adjacent to software development.

This usually takes about 5 to 7 years, in my experience. After about that much time working in a focused way on software you're probably pretty "senior" at dev work, without a whole lot more to learn in terms of just getting better at coding.

The author doesn't quite fit that description. This sounds more like what happens at lots of high turnover "fast paced" startups. As people come and go from the group they find themselves in a situation where their 1 or more years of experience is allowing them to significantly outperform those with 0 to 6 months of experience. Maybe high turnover or rapid growth at the employer makes it easy to get promoted to "senior" because there are so many other truly very junior people around.

Not to belittle the author at all. Sounds like a smart guy and I admire his enthusiasm and spirit. Just sounds like crazy-startup-battlefield-promotion "senior" rather than the more rigorous definition of senior some of us might be thinking about.