You do not recall correctly. There is more than 500K SLOC of test code in the public source tree. If you "make releasetest" from the public source tarball on Linux, it runs more than 15 million test cases.
It is true that the half-million lines of test code found in the public source tree are not the entirety of the SQLite test suite. There are other parts that are not open-source. But the part that is public is a big chunk of the total.
> TH3 Testing Support. The TH3 test harness is an aviation-grade test suite for SQLite. SQLite developers can run TH3 on specialized hardware and/or using specialized compile-time options, according to customer specification, either remotely or on customer premises. Pricing for this services is on a case-by-case basis depending on requirements.
It is true that the half-million lines of test code found in the public source tree are not the entirety of the SQLite test suite. There are other parts that are not open-source. But the part that is public is a big chunk of the total.