Yup, that is the very self-same article that I read.
"The ignition temperature for Pyrodex is approximately 740 degrees and the ignition temperature for Goex Pinnacle is around 700 degrees. As a result, for some guns, black powder substitutes are not an option. As noted in the article dealing with true black powder, Hodgdon recommends that a small amount of true black powder be used to help facilitate the ignition of both Pyrodex and Triple Seven in Flintlock guns. " -quoted from Muzzleloadingshotguns.com
No matter that 4F was placed in the pan, the flash is simply not hot enough to flash over the high ignition temp compounds such as Pyrodex or Triple Seven. I accidently charged a flintlock with Pyrodex once and eventually after trying every trick in the book, including grinding 4F powder into flour and pricking it into the touchhole with a wire, we had to drive b ack to town to get a ball puller to manually unload the gun. And it's not just me, I've contacted Hodgdon to ask about how to use Pyrodex in a flintlock and they simply said without a duplex load I was in for nothing but agony. (My community has a law against the sale of black powder within city limits, so I was looking for a replacement)