The Zoox acquisition offers are all back-weighted. There are several posts on Blind recently disclosing back-weighted new-hire grants. The mean tenure time at AMZ is 2 years and most engineers I know do NOT vest 50% at the 2 year mark.
The mean tenure time is a terrible measure for any company that is growing at the rate Amazon does. If you double your team every year, the average tenure is going to be really low even if not a single person quits.
