I might be wrong here but I think Puppet DB is more like an eventually consistent database that stores the last known state of potentially thousands of machines

I might be wrong here but I think Puppet DB is more like an eventually consistent database that stores the last known state of potentially thousands of machines that may not be reachable or even turned on. The use case is totally different. Puppet runs on each machine "in isolation", and before Puppet DB, the agent had no way to know about anything external to the local machine. It can be used while assembling the manifest that gets passed to a machine, but when the puppet agent actually runs on that machine it doesn't ask Puppet DB what state the local machine is in, it just looks directly at the resources because they're local - there's no performance penalty.