I agree.They know exactly who it is. They knew probably within an hour. The delay is so they can come up with a narrative that they are vetting. That's why their story keeps changing, and they aren't releasing any details.
All guests are vetted, and everyone is checked in. There are cameras literally everywhere, as it is one of the most secure places on the planet. They use all kinds of means to keep track of everyone. They have real-time monitoring with facial recognition (same technology that was used to track down all the "guests" at the Capital on Jan 6th, who had not been vetted or signed in.)
If it had been a grenade or any kind of threat, the secret service would know in an instant. They would pull video from the room, and there is probably a list of everyone in that room they can pull.
They can identify everyone at all times in the White House. They know who entered what room at what time, and when they left. No question about it, I think.
If it was any other substance, we'd have known who and when it was left there within hours.
My guess is that someone or several someones will be "reassigned" or fired for having disclosed that the substance is cocaine.