It's not a check box, its a button choice ("Share diagnostics" "Don't Share")

It's pretty explicit in intent.

My mistake, I misremembered. s1mplicissimus makes a very good point. What I do remember is it's designed to capture telemetry opt-in of people who aren't savvy and just click next, next, next.

Usually, there's a default blue bordered button and a non-highlighted grey one. Any chance the default blue button says "Share diagnostics"? Because that would still make it an opt-out