Presumably it would be validated by the destination, but that doesn't matter. The reason spoofing matters so much for volumetric DDOS attacks isn't due to spoo

Presumably it would be validated by the destination, but that doesn't matter. The reason spoofing matters so much for volumetric DDOS attacks isn't due to spoofing the traffic sent directly to the target - it's because the target is spoofed as the source address in traffic sent to third-party amplifiers, that respond to the target. If the third party amplifiers in this scenario can validate that the traffic is spoofed, it cuts out amplification attacks.