While the Monzo FAQ on their website is very vague and does not give details, the processes they use to ensure KYC compliance entail checking various UK based sources to prove your ID and residence. They do not check foreign databases nor do they use companies like IDnow so they do not have adequate systems in place to ensure KYC compliance for non-residents. However they have plans to use a similar company when the current accounts are launched so this will enable them to expand what applicants can be accepted in future.
Also the card is currently run under an agreement with Wirecard and while that agreement contains age restrictions it could potentially contain geographical limits too.
Various staff like Naji have said before to interested enquirers from outside the UK that they can not get a card and will have to wait for Monzo to expand into other countries, so just having a holiday for a few weeks here does not legally alter the position that they are not UK resident.
Granting a card to anyone who provides a UK address makes a mockery of the current restrictions as any holiday maker or anyone with a friend here could apply, yet up to now there have been restrictions if for no other reason than to control the quantity of applications.
There are also national and international embargos on the provision of financial services to citizens and residents of certain countries and Monzo will have to have systems in place to ensure that compliance prior to changing their current UK only rule.
Monzo have even been strict in their interpretation of UK before with some discussion of weather to extend the right to get a card to people in the Crown Dependencies. They would not have bothered with that discussion if they were freely dishing out cards to non-residents