That is not something that’s necessary for all CQRS systems, but maybe is something you’ve heard for the subset that people call “Event Sourcing”? There it’s a design goal that the system only records events that are occurring, so there’s no domain level validation that can be done on the command path - the user pressed the button whether we like it or not, so to speak. Whether the event has the intended effect is worked out after the event is recorded.

But there’s nothing in the more general idea of “separate reads from writes” that mandates “no validation on writes”