Yes, but for a volumetric attack, it doesn't matter if you know the source IPs. It just fills your pipes until legitimate traffic can't get through. (This wasn't a volumetric attack though, which is why it would have helped.)
But most volumetric attacks are reflection attacks, which would be impossible if BCP-38 were implemented everywhere. Direct non-reflection volumetric attacks of significant magnitude (say above 40Gbps) are almost non-existent.
