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.