Specifically, they're not permitted to speak to other employees about the apparently confidential work they're doing, which presumably was mentioned to them when they went to the job interview round about the same time they were advised they'd be missing out on a few other social events. Even the original author doesn't seem to be suggesting they're being held and silenced against their will...

You can call a thing confidential, prohibit speaking about this thing, and prohibit speaking about the fact of prohibition. Therefore declaring something confidential is like putting it into opaque box - we can not figure whether the thing in the box exists and is really confidential.

Therefore they can not prove that they've segregated employees to prevent confidentiality breach, and we can not prove that they segregated employees to prevent people becaming aware of the class society. I don't know how to resolve this problem. We just don't know, so it's a question of trust.

I don't trust that Google has a big amount of highly confidential but unqualified data-entry labor, and they can't prove otherwise unless they lift the confidentiality wall.