Forcing all dates to be cast to a single time zone (UTC) complicates workflows when you’re on the other side of the globe. Many DBs have added a timestamp offset type for this to keep the data in the offset it occurred in.
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.
They sell several niche things, for hopefully hundreds of thousands of dollars, to just a few people each. TH3 is just one instance of the general pattern.