There are 3 people on the planet who can make changes to it and one person who can work on their custom made source control system. Single points of failure are not sustainable.
Well, it's been popular for 20 years, so that sounds fairly sustainable to me. A lot of projects with many more contributors have come and gone in that time period.
Either way, that doesn't make it "closed source" like you said in the other comment.
> There are 3 people on the planet who can make changes to it
Anyone on the planet can make changes to it and distribute those changes. The only thing those 3 can do that the rest of us cannot is get their changes into the copies distributed at sqlite.org.
Either way, that doesn't make it "closed source" like you said in the other comment.