It's not really time. Seniority here is more of a quality than it is just something determined purely by time. So "Senior Software Developers" are those with some of the correlated skills in the article.
The article uses phrases like "As a developer become more senior, they become more independent.", which indicates they view seniority as the independent variable.
Yeah, it looks like seniority is the dependent variable. It also looks like it's the monotonically increasing variable. Seems like it should be on the X axis.