I think it's just that briquettes are more likely to give a long stable burn than lump is, because of their regular size & shape. Lump tends to burn faster and hotter, which is great for grilling but not so great if you want a 16 hour smoke. I suspect the varying sizes would make for a bit more variation in temperature as the coals burned down and settled, but I don't have direct experience to support that.
That said, I can't think of any reason not to use lump if that's what you have other than burn time and ease of maintaining consistent temperatures.
I was smoking a pastrami-cured eye of round last night in a WSM 18.5 and all I had was lump, so used it and it worked fine. The smoke was only a couple of hours though on about 1/2 a chimney of lump, no idea how long it would have lasted. Bottom line is that one of the big attractions of the WSM is their ability to maintain consistent temps for a long time, and briquettes work better for that than lump.