Also worth mentioning are VeloxDB and RavenDB, both written in C#. TBF, I haven't used any of them... but aware they exist.

C# is pretty powerful and capable of lower level usage, such as in the examples given... not to mention a pretty nice interop with C-style libraries. It looks like the intent here might be a custom database engine for service integrations... not necessarily a full rdbms in and of itself.