This is related to their app not their website.
Same applies to their app.
Doing retargeting for when (a) someone downloads their app but doesn't signup and (b) someone is a customer but has low engagement i.e. is likely to churn.
That may be true, but I still think it's a low-effort way for them to do it, and I expect better from Monzo than this. Plus it tracks people regardless of if they fit in a) or b) - neither of which is the case for me.
They have the Facebook Pixel installed likely to do retargeting advertising when a person visits their website.
It's one of the most effective methods so it's very common to see it everywhere.