The license switches to Apache after four years.
To complete the metaphor: “I will only eat meat on Wednesdays for the next four years, after which I will be 100% vegetarian.”
As someone who is annoyed anytime proprietary-licensed software tries to capitalize on FOSS terminology, I concede that being immediately source-available with a guarantee of being OSS within a reasonable timeframe is still better than we’re likely to get from the majority of projects.