I find it ironic that they claim "[s]ource code transparency is an absolute requirement for software solutions like Bitwarden" on their website yet they require SQL Server 2017, a completely proprietary RDMBS.
All of the software written by "Bitwarden" is open source. The fact that it uses some pre-existing propriety software doesn't change that. If it did, then that logic could really be extended to any piece of software written for Windows.
You can write open source code for Windows all day long and it doesn't change the fact the your code is open source.
However, to claim that you need transparency for your security product and then build it on top of a proprietary storage engine is incongruent.
Also, there are many server implementations other than the official one.
They rely on it to store encrypted data.
If I recall correctly they use an authenticated aes cipher - and the Free software part can verify that correct data is read back (ie: it decrypts and authenticates).