To summarize, instead of using one of the OSS licenses, the copyright holders simply declare the source to be in the public domain. In order to preserve that status they don't accept patches unless you submit some signed document that you agree with that.
To make things more complicated, they also use their a relatively niche version management system instead of git. Which would complicate making contributions (if they accepted them).
There's a popular fork that fixes all of these issues: https://github.com/libsql/libsql It is MIT licensed, on Github, and open for contributions.
Kind of a weird legal situation for a popular project like this that so many people depend on to have. Not judging; but it is odd. Seems like a lot of wasted efforts between users, would be contributors, and the people that forked this thing to address all that.