there doesn't appear to be any timeline announced here. the article says the "initial investment" is expected to be $100bn, but even that doesn't mean $100bn this year.
if this is part of softbank's existing plan to invest $100bn in ai over the next four years, then all that's being announced here is that Sama and Larry Ellison wanted to stand on a stage beside trump and remind people about it.
Seems like you nailed it.
The literal first sentence of the announcement is:
> The Stargate Project is a new company which intends to invest $500 billion over the next four years