Can Immudb work in a decentralized network while remaining secure from attacks in such networks or is Immudb meant for centralized systems if so I think you cannot compare it to Blockchains. Maybe a better comparison is Git.
immudb is not meant for public decentralized networks, although it might be possible to use embedded immudb to build a public blockchain... but that's a different story. immudb server is tailored to provide a database where any tampering will be subject to detection by any single client application consuming its data.