In my experience, levels are more a question of delivery and trust than of technical skill. If a Senior is off down a rabbit hole, that's them doing their job. It's almost a defining feature of the level down (SE2, or local equivalent) that if they're off down a rabbit hole then something's probably gone wrong somewhere: if you could trust their judgement, they'd be a senior already.

(Yes, that's circular and overly reductive, don't take it too literally)

Where I've worked, "senior" has also meant being a final authority: more junior folk will come to you with questions about your team's codebases, and more senior folk will too. You might not have anyone else to ask that kind of question of.