Complete a risk assessment with the HS Committee. From experience I don't see the harm. As one of the controls you can add ....take laptop and personal only if safe to do so. Keep it practical

Regardless of what you put down in a policy, people will grab their personal items - so they will be in breach of the current policy.
I'd rather set reasonable expectations (e.g. purse, wallet, phone, keys. Maybe a small bag?) Set a limit around something that can be held in one hand, so they can still have a hand free for the stairwell and have to be able to take it within 1 min of alarm to get out (or less?). Then explain why they should only grab those critical items and why they need a free hand.

Yes, carry out a risk assessment. As part of that carry out an unannounced evacuation drill and then survey people to find out what they took or went back to collect (!).
Some anecdotal thoughts to add to the discussions.
A long-time friend is at Canterbury University. She described how after the worst earthquakes she always took her keys, wallet and phone with her when going to the toilet. She had experienced earthquakes while in the toilet.
If necessary, I could log in to my Victoria University of Wellington account using my wife's computer (which lives at home) including everything for teaching but would lose access to other files on my computer. They're backed up in three different places but getting a replacement computer would take time.

I am in Christchurch and have had this conversation with FENZ (they relented). When our people evacuated for the quakes they left behind keys, phones and wallets and then spent the next few weeks trying to manage their lives without their stuff as they couldn't get back in the building (the building was stickered as opposed to munted).
Our current guideline, and what we practice our drills with, is that it is OK to grab that stuff but you must be hands-free upon exit, bags must be closed and secure over your shoulder (hands bags over your shoulder and securely tucked under your armpit), and we must maintain an acceptable building-clearance time.
This is working so far, and it actually helps in the battle about people bringing their coffee cups with them because the conversations are better focussed on being hands free and what that means.
We would be fighting a useless battle to stop people from grabbing that stuff so it is by far better to provide guidance and encouragement to do it to a standard that keeps everyone and compliance happy.

We maintain if they can take personal items with then do so, but they are not to deviate from evacuating to get personal items.