They're starting at Square just like they would start at any other job. Thus they will stay at Square if it is a better job for them (for whatever reason) than they could otherwise get.
They definitely could shop around and find a new role (if they can find another company they want to work for in Atlanta) but it's unlikely to come with as healthy compensation as that.
Plus if you like your team you can keep working together.
I'd say that's accurate to describe as "golden handcuffs", since they (probably) can't just walk down the street to another company and get that offer matched.
(Hope this isn't coming across as combative — if we're gonna have a discussion of semantics, I'm trying to nail down where you're coming from :) )
The "tightness" of the handcuffs is, roughly speaking, the % of your motivation to work there based on a previously accrued reward vs reward that is earned day by day.
With powerful enough golden handcuffs the company could reduce your salary to zero and you would still stay (though perhaps be pissed about it).
(not coming off as combative :) )