Yes: sqlite3 only allows a single open write transaction per database file. Whether this matters or not depends on your application. For a large class of applications, this isn't a real issue at all. By way of example: until a few months ago, we were concurrently merging state updates from tens of thousands of VMs across our global worker fleet into a single SQLite database. As has been said across this thread, SQLite works well as a backend for read-heavy applications. A great many CRUD apps are read-heavy!
If you want more concurrency, you can break your schema up into multiple .db files. This sounds onerous but SQLite makes it very easy to do.
That's great it worked for you, but that's not a website backend. It would be foolish to run HN on SQLite for instance. I would go as far as to call it a vulnerability in such context, due to how easily it could be DoS'd.
Maybe you don't understand SQLite's locking issues? Having a bunch of different files that can be written to concurrently sounds like a better solution than SQLite for a social network backend.
If you want more concurrency, you can break your schema up into multiple .db files. This sounds onerous but SQLite makes it very easy to do.