The medals were 23K, so about 95.8% gold.
But even if you could pound them into unidentifiable blobs, how would you keep them safe? The Germans weren't looking for medals in particular, they would have taken anything made of Au. "No gold shall leave Germany", etc. etc.
But without the identifying marks, how would they know where they had come from or who had sent them? Wasn't the overriding concern the protection of the two Laureates?
I also think hammering the medals would have been a good and fast first step to take to make them unidentifiable. However the gold still looks like gold and it would have been confiscated. Their goal was also to hide it in plain sight. As mentioned in the article, they were sure that even burying gold somewhere wouldn't help.
In the Italian Job, the thief of thieves fenced the gold bars, but was discovered because of the image of the lady printed on each one. Bet that'd be easy to pound out.