Strange that may seem but let me exaplain my logic based on this picture:
ok, so we have all seen this and have finally accepted GW are finally giving the guard some love! So I hear you cry why is this a Storm Troopers army?
Three reasons jump out for me:
first the codex, not only is it online only but its a mere 72 pages, a true IG coded would be equal if not greater than the exsisting SM codex and importantly everywhere!
second the names, if i remember my old fluff I am sure that militarum tempestus is the name of the military academmy in the tempestus sector where the storm trooper regiments and commissars for the segmentum are trained.
third and finally its the name of the unit "Militatum Segmentus Scions" to me this harkens back to the old background where the storm trooper regiments were taken from the segmentum scholam where the orphaned sons of military officers were sent, in otherwords their scions.
so those are my thoughts, could it be GW are giving us a storm trooper army in a similar vain to the defenders of terra codex? could it be we are getting some shiny new models to the already great range we have that are able to be used in their own right?
let me know your thoughts and interpretations!