In a 2021 podcast interview[0], Dr. Hipp noted they've sold zero copies of the TH3 (extensive, proprietary) test suite in SQLite's history.
By the time TH3 was added in 2008, SQLite had gained a fair bit of traction across multiple industries. Though I totally agree that the comprehensive coverage is a leading reason why they've been so stable over the last ~20 years.
So indirectly, TH3 is why they (continue to) exist and (are able to) make money, but it isn't a direct line as one might assume.