The truth: Eenie meenie miny moe
The truth: everyone knows who the underperformers are.
AFAICT, companies are both jettisoning entire departments related to bad bets (or bad business models) and preparing for anticipated recession-level revenues.
Skills are not perfectly fungible. Managers making layoff decisions based on the roles/skills they need to keep and what budget they can afford. This necessarily means that some underperformers will be retained and many overperformers will be cut.
Sure some employees on PIPs or otherwise considered low performance may be cut regardless of role. But more typically projects get cut/defunded (whether in engineering or elsewhere) and those roles are eliminated or cut back. A good performer may have an opportunity to find a job elsewhere in the company but, in my experience, that's pretty hard given that any open slots were probably also closed as part of the layoff and everyone is pretty distracted anyway.
It reminds me a bit of the argument against massive conspiracies to demonstrate how many people have to perfectly as an organization (leaving no evidence behind) and then have to stay quiet indefinitely. The larger the accused conspiracy, the more likely information is to leak.
Furthermore, having worked in big tech, most of the roles that are getting filled are essentially "busy work" - i.e here is a entire proprietary build system, barebones service and deployment pipeline, all you have to do is add logic. Someone who has worked in this type of environment isn't going to succeed at a startup where you have to build systems from scratch.