This is actually a security feature, as James explains here -
it could potentially be replaced / relaxed by a feature like the one that he describes or through this feature, from Monzo’s extraordinary ideas board -
it’s also worth noting that Monzo is reasonably relaxed about this, as you can currently log into your account from an iOS & Android device at the same time -
that also suggests to me that they will make this rule less restrictive in the future
Lastly, there’s been some more discussion about this idea in the posts above, in the topic that Tom’s shared with you & here -