Why the bomber could get through but not fulfill Douhet's promise

Did the difficulty of detecting and intercepting bombers in the 1920s justify the claim that the bomber would always get through?

It supported the penetration argument, but available bombers lacked the payload needed to deliver the devastation Douhet predicted.

The claim that bombers could penetrate had a sound period logic before radar-supported fighter defence matured. Penetration did not guarantee the promised effect, because contemporary aircraft could not carry anything close to the enormous bomb loads imagined by Giulio Douhet. A plausible observation about interception therefore did not make the wider promise of war-winning bombardment technically sound.