If the story was of great importance, if Wrede had said something controversial, then it certainly shouldn't be accepted on Macdonald's word alone. Currently we have no other primary source to support it or contradict it. However, it is pretty harmless and if it is just being used to add colour to a narrative then I would say Macdonald's version is adequate for that, but it should be attributed so people can make their own minds up.
Macdonald did accuse Napoleon of dodging fire at the battle of Hanau (people don't like to mention that but I think it is why he gets the 'unreliable' tag): in that case the standard of proof required would be rather higher and even I would say that Macdonald was probably putting the worst possible construction on events*. To quote him as evidence for that would be quite unjustified unless there was strong corroboration. So far I haven't found any.
* In that section he was still seething over the Lindenau bridge disaster