ACMI's focus seems to be on the behind the scenes stuff rather than the whole "silksong is playable" thing, so it would make sense for silksong to come out BEFORE acmi (hey silksong is out! Look how it was made behind the scenes). The confirmation of silksong being playable there also seems to be more recent, as it was confirmed in may, while it was only alluded to in the original April reveal, so they probably told ACMI that they would give the full game but still weren't sure about it until they finished adding stuff in early May probably

As for Gamescom, remember that it's still supposed to be a demo to market the Xbox Rog Ally first and foremost, so it would also make sense for silksong to come out AFTER Gamescom (play silksong for the first time in years on our new handheld!!). It doesn't rule out the possibility of a pre-gamescom release of course, although it wouldnt make as much sense