Well it all depends on how one define the term IBL and if it should be true for all traits or just some. I do agree that through most of the C99 lines I have seen there’s been a few things that seem to be dominant, like early maturing flowers but I have also seen variations in flavor, smell and high which to me makes it not true for those traits. Not all C99 have a tropical flavor/smell but many do.
C99 is supposedly a Jack Herer as the bud where the seed the mother (the recurrent parent in MrSoul’s backcrossing schedule) came from was allegedly Jack Herer from the Sensi Smile Coffee shop. It’s no way of knowing if the pollen came from the same female plant, another female in the same line, a male from that line or if it was totally unrelated. The genetics of Jack Herer has never been disclosed by Sensi Seeds but it’s allegedly a mix of Skunk#1, Northern Lights 5 and Haze. Nevil said it was the same breeding pedigree (not the exact same plants) as his Super Silver Haze which is (NL5 x Haze C) x (Skunk#1 x Haze C). So even if it was only Jack Herer involved it has at least three different lines in it’s making.
I do agree though that C99 usually is worked much more than a usual line and if people also are continuing that work down the filial generations it would sure be more inbred but also the result depends on the selection each step.
I still have some quotes form almost 20 years ago when Soul talked about his second release of C99 which was the P97 or 4th backcross. He said that the results were counter intuitive and gave more variation than the P94 or 3rd backcrossed generation did. Maybe I am just too hard on the definition of IBL but this discussion was up already back then when Soul called his C99 true breeding which set some people off. Again I agree it seem true for some traits but even the P94 were not all the same.
I have also seen Chimera lay out a bit more complicated genetic models on what happens if you filial breed the C99 and it will segregate more but then again applying selection is key. His model, as far as my layman comprehension of genetics go, was assuming using all plants which I assume people have not done when making their own versions of C99. They applied their selections on the line.