Because people have a hard time for some reason wrapping their heads around what a "Space Marines Detachment" is. Luckily enough, CRIMSON FISTS is one of the chapters on the list that allow it to be a Space Marines Detachment.
So yes, a Crimson Fist army counts as a Space Marines Detachment for the purposes of any rules that care about that, and you can use the Warlord Traits etc because it is also a CRIMSON FISTS detachment as defined by the supplement.