There’s been a bit of discussion on this under the different names of 3D Secure. I think this is the latest word from someone on the Monzo team.
Many cards that “skip” 3DS (especially credit cards) are actually using techniques such as browser fingerprinting to determine if the transaction is likely to be legitimate. I would love it if Monzo could use this same technique up to a point and then jump to something like a slide to verify within the app when the transaction is more questionable.