Maybe you could enlighten the rest of us then. According to your favorite definition, what does senior mean, what does seniority mean, and what's a term for someone who knows what they're doing?

This is not necessarily true in practical terms when it comes to hiring or promoting. Often a senior dev becomes a senior because of having an advanced skillset, despite years on the job. Similarily, often developers who have been on the job for many years aren’t ready for senior because of their lack or soft and hard skills.