It may well have just been a general recommendation based upon ignition of the charge. A lot of the early brass shells were designed for black powder & used a relatively mild primer which was adequate for black but not for some smokeless powders. Parker may well have just thought it advisable to recommend not to use smokeless rather than to sort out which ones gave adequate ignition & which ones didn't. Also if the top wad glue job didn't give an adequate pull about equal to opening the crimp of a paper shell you could end up with a squib load using smokeless. With black as long as it kept the shot from rolling out of the hull you were OK.
