It’s not true that SQLite is tiny. I don’t know what portion of the test harnesses you’re seeing in the GitHub repo you looked at, but a portion of SQLite’s tests are proprietary. [0] The size of the SQLite test code is legendary, and exceeds that of the program code by an order of magnitude.
OP was clearly talking about team size and change velocity, not code size.
5 contributors, with over 70% from a single person is indeed small.
That’s totally fair, but insofar as the number of commits matters at all, the number sited in the grandparent post for total commits is deceptive. It’s a fraction of the number of commits that team makes.
You don't need complicated branch refactoring tools when you're only sharing with a handful of people. It's like working with RCS back in the day: history is still linear and no one needs to worry about cross-ports between feature branches.
[1] Ironically done using a github mirror, because those are the tools I know and they're fast.