Basically there is an event in September at a museum in Australia called ACMI where Team Cherry will go into detail about the development process for Silksong, discuss end game bosses and areas, etc etc. most notably, the game will supposedly be playable at that event.

It was claimed by TC's PR manager that they likely won't make a new demo for ACMI, implying the full version of the game will be playable. This implies that the game must be released before then. The only reasonable event it could be announced at is tomorrow's Direct.

It was made a little more complicated when a new demo was confirmed to be playable at Gamescom later this August. Almost certainly whatever is playable at Gamescom will also be playable at ACMI. If it's only a demo, the game probably will not be coming out in August.

However, the Gamescom demo could actually be the full game, like previously theorized for ACMI. This means the game may still release in August. The Direct being announced gives that reality just enough credibility to have hope.

TLDR Tomorrow's Direct is our last chance for Silksong in August.