And it happened a decade ago, as evidenced in the image.
If this makes no sense, then I'll elaborate. This sentence is the first one from "Horus Rising", the first publication in the Horus Heresy series of publications from the Black Library. This year marks its 10th anniversary. The entirety of the Horus Heresy from Isstvan III through to the Siege of Terra takes roughly the same amount of time in the cannon. Hence the reason for showing this photo is a demonstration that it has now been ten years since we started and we're nowhere near the Siege of Terra.
I for one sincerely hope that it is not another decade before Black Library advances the plot sufficiently so that we can see the traitor fleet arrive at Terra for the final confrontation. Please don't drag it out - the "filler" novels can always be written later.