I saw this earlier - its an interesting (and probably inevitable) development. I agree with Monzo's position that technology (and their core banking) is a competitive advantage. I think the existing banks recognise their positions of strengths and weaknesses (e.g. strength: cash, existing userbase, 'trust', brand, weaknesses: agility, technology, legacy systems and processes)
If the banks see their own delivery capability (as well as technology) as a position of weakness then its actually better to acquire a new core/test it from a partner rather than build your own. I suspect many of the banks will be considering this and are watching to see which emerging cores are proving the best.
This will be an interesting existential question for Monzo in the future. Should they consider selling/leasing their core banking system to other banking providers? (a pivot ala business model canvas where you look at different business models from your assets). If yes, lease the core then should you also sell data insights?
Edit as another thought occurred to me - in a way, I think a more interesting challenge to traditional banks than what core/experience/data insights layer they use is what will happen to their actual financial products (e.g. savings accounts, investment accounts, mortgages, loans). Monzo's vision as a marketplace bank is that it will open up to specialist providers who provide the best product - what will banks do? I suspect we might start to see more packaged together products offering more benefits than individually going to providers (e.g. trying to make a better case for why you should have your mortgage, savings account from the same 'provider' as competition against a marketplace)