SQLite shifted to Fossil in stages. Documentation moved in 2007. The TH3 and sqllogictest test suites started out in Fossil in 2008.

I wrote Fossil specifically to support SQLite development. If Fossil does nothing else other than support SQLite, then it is a success. Any other use of Fossil is just gravy. That we were conservative in moving the main SQLite source code into Fossil does not negate that fact.

We do also use Fossil for dogfooding SQLite. See, for example, item 15 on the release-testing checklist: https://www.sqlite.org/checklists/3230000/index