The requirement only applies if one can create the account in the app. At least here in Finland that is not an option in any of the local banks apps I have used.

You might think that at first, but the distinction can be made for data as well. The argument goes, when is your data, not really your data? How about, when it's actually my records, of your use, of my system.

If you allow such a construct, then "deleting your account" could mean, your immediate personal details (or perhaps even just your access credentials) are erased in some fashion, but nothing else.

This is how legislation like the GDPR gets motivated, of course. The Apple guidelines reference "usage data" elsewhere, and I imagine that's for similar reasons. The deletion clause itself, rather notably, doesn't.