[Someone]

High cohesion and low coupling are good, and easily testable correlates with that, but having lots of unit tests doesn't imply that the code is easily testable, and not having unit tests doesn't imply that code has low cohesion or high coupling. It's even possible code needs many lines of testing partly because it lacks in ease of testing.

In the case of SQLite I think it mostly is because of hard work to fulfill the ambition to deliver a robust project and because of the existence of fuzzing, which can automate test generation.