I don’t know why you think that’s true “in general”.
If you have a team with a senior member it, in general, what would you expect that team member to do? How are they different from the other team members?
They are not senior because they’ve been there the longest. That’s stupid. That just makes them the oldest worker, not the most senior.
I expect them to have more responsibility for making sure the team delivers whatever it is they do.
If the team does tax forms, then they’re more responsible for ensuring the forms are correct, and timely, than say, a junior accountant on the team.
If the team produces code, they’re responsible for making sure the team produces the correct, maintainable code in a timely manner.
If the team is two people, maybe that means “write lots of code”, but if it’s six people, it’s much more likely, in general, to be, making sure the rest of the team is doing the right thing at the right quality. Mentoring. Checking. Meetings. Documentation. Protecting the team from politics.
Why? …because seniority is about responsibility, and if all you do is write good code, you’re not accepting the responsibility for anyones effort but your own.
Obviously, it’s not solely your responsibility; you share that responsibility with the other team members depending on seniority. A team lead, for example, and other seniors.
…but, I don’t care how many years experience you have: 0 responsibility for anyone other than yourself makes you a junior developer.