I don't think you understand the problem. The issue is that some of these people might actually believe they did something wrong, or didn't measure up. That is the problem. They should just be honest.
There's no law against laying people off because you hired too many people and need to downsize. They are using performance as a reason because they think (and in many cases, they'll be right) it will subdue the person being laid off from a position of anger or resent, to a position where they're upset with themselves for not measuring up.
It's a really bad way to do this, for the person being laid off.
So, yes. Asking about the fictional performance metrics to at least make them feel a little uncomfortable too is completely fine in my opinion.