It clealry shows that such flavoured stories found in a biography of Napoléon cannot be taken seriously without cross checking.
In his biography about Wrede - Heilmann, tells another story, Wrede was close to the Bavarian artillery battery Wagner, then his horse was shot under him and then he got very slightly grazed by cannon ball above the right ribs - Major von Zoller commanding of the artillery brought him back some steps to the rear till an ADC of Wrede took care of the wounded general, Wrede should have said to Zoller that he should take care that the Bavarian troops are not dishonouring themselves - no mentioning of MacDonald at all.
I ask myself - why should MacDonald be along Wrede, he was much in need to be with his units and not Wrede, who was behind him.
A man wounded in the rib cage would also unlikley to shout a lot around on a battle field.
The Bavarians also likley would have mentioned this incident with MacDonald in case it has happened.