I think it's probably a bit of both. A good senior engineer may pick up a task and look at the system, seeing hacks duct taped together with kite string, and have the choice between "doing it right" (aka rewrite/refactor) and getting shit done.

They wouldn’t have to fix the entire codebase to be doing the right thing. That approach would be more likely taken by a junior who would soon find they are fighting a losing battle. The senior engineer would accept the state of the world and consider how to make small, deliverable, manageable improvements one PR at a time, to improve the code over a longer period of time without breaking the system.