Makes sense to me... leaving to start their company with Google being an investor.

Clayton Christensen [1] says (paraphrasing) it's a good idea to spin-off (or invest) in a startup that you've a say on, before the ecosystem sprouts a seemingly-non-entity and gently disrupts your market.

Interestingly (diff strategies i guess) Apple tends to acquire (Q.ai) rather than invest/venture (as google in OP).

[1] The Innovator's Dilemma: When New Technologies Cause Great Firms to Fail.