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.