It's a well established fact that the charcoal source can make a significant difference in the potency and burning characteristics of homemade black powders. Choosing to use pine, cedar, balsa, willow, alder or just any other plain hardwood charcoal can cause a black powder to be either more or less of a success from what I've read about making it.
So unless someone specifically knows about the actual effect of the use of unbrunt rye straw as a charcoal replacement (or in addition to), then the little that was published in the Shooting Times article may be accurate as far as unknown facts are concerned.
If they added smokeless to it or other compounds to increase its power output then its not BP anymore.
There was no mention that smokeless powder was added to make the German brown power, but only the rye straw. But even if the resultant potency of the brown powder was being exaggerated, the question still remains about its existance as a non-synthetic substitute, and if it was possibly better than regular black powder or was flawed in some way that keeps it from being produced today.
I obviously don't know many details about it, but it's not the first reference to there being a brown powder. But since it existed and this discussion about substitute powders includes why they are called synthetics when they might not all be, who does really know about it?
Could the simple substitution (or addition) of the rye straw have led to a better, an equivalent or a worse brown powder being created or not?