Thinking about the practicalities of this.

If you arrive and there's a queue, then it's easy enough for a steward to check your covid passport whilst you're waiting to get through the turnstile and it won't delay you getting in.

If there's no queue, then the few seconds it will take for a check won't delay you.

It might not be practical to check everyone, but a large enough proportion can be checked to provide both reassurance and deterrence.