How do I prevent SQLite database locks?
That page you linked, besides being quite old, talks about accesses from the same process through the same database connection (or through multipe connections in shared cache mode, which you should not use).

When not in WAL mode, multiple connections can read from the same database, but a writing transaction is exclusive, i.e., no other readers or writers are allowed.

In WAL mode, a writer and readers do not block each other, but there is still only one writer allowed.