I think the distinction in this case would be:
Monzo decides to exclude US citizens from having a current account because of an assumption that it would be too costly or bothersome to comply with FACTA etc. = potential discrimination under the EA 2010
Monzo does the calculations and puts together a report showing that it would at this point be prohibitively expensive to provide US cotizens with a current account = probably not discrimination as above
Don’t forget there are lots of listed buildings, train stations, and other public places that are 100% in no way compliant with the old DDA accessibility rules, and continue to get away with it, because they can show it would be too much money to retrofit them and have an alternative access plan in place (whether paying for transport to/from an accessible station or having someone bring items to the ground floor for someone on request). So exceptions exist and it’s a matter of how they’re addressed and dealt with that makes the difference.
(Please note my main knowledge of direct/indirect discrimination comes from an employment law setting and is a few years old so my mad case law skillz may be rusty)