The point isn’t that depression has no biological cause. The point is that there are probably numerous biological causes. To strain the analogy, there are probably dozens of different depression gremlins. Maybe even a depression goblin, and a depression troll.
Saying that you have “depression” doesn’t give you an understanding of what’s causing it.
We have a baby with colic. What does that mean? It means the baby cries 3 hours per day for 3 days a week for at least 3 weeks with no other explanation. That’s it. It’s not something you can point to and say oh that explains it. That’s the cause. It literally just means there is no explanation.