It isn't a myth. Sources disagree on this subject and I'm not surprised that a German source would.
The question is was the 'devastation' carried out? I don't think so. What has to be done is find the order in Napoleon's Correspondence and find out if it actually happened.
It seems to me that if it did, people would be crowing about it because Napoleon allegedly did it. I haven't seen that at all.
Further, sometimes Napoleon had a knee-jerk reaction for a course of action, such as his instructions to Davout to deal with Hamburg in 1813, and after he thought about it (and in this case Davout advised not to do it), Napoleon's good sense would come through and he would cancel the order.
All we have now are two secondary sources that disagree. And I'm not surprised that a German source from 1852 shows this.
So, in order to find more information, Napoleon's Correspondence needs to be looked at and the incident investigated. Again, I haven't seen anything that supports the proffered document.