The Duke of Richmond

I was reading the Wikipedia entry for the Duke of Richmond and it says that he was in command of a reserve force for the defence of Brussels during the Waterloo campaign. Was this actually the case? My understanding was that the Duke had been passed over for military command and was rather bitter about it. He is also famous for having appeared on the field of Waterloo wearing civilian clothing, and it seems odd that he was not in uniform if he was indeed in command of a reserve.

Anyway, can anyone shed any light on this? Did the Duke have a command or is it a mistake in the Wikipedia entry (the same article states that he was an observer at the Battle of Quatre Bras, which I am fairly sure he was not, so I am suspecting the latter).

https://en.wikipedia.org/wiki/Charles_Lennox,_4th_Duke_of_Richmond

Stephen