Normal people can't just hit their bank for an APi key, it's designed for regulated third party services and banks to have a common API to talk to each other that has to comply with the spec (I think, at least this is my understanding of it anyway)

note: I'm aware some of the challenger banks here (Starling etc) have a developer API, but that's their own offering

Open Banking means you can access your own banks API, however if you have a lot of customers and you need to access lots of different APIs from different banks then you use an intermediary 3rd party, e.g TrueLayer and you use their API to access the open banking API of the customers bank.

I don't think that's entirely correct. Open Banking means that all participating banks allow access via the same API, which is documented here:

https://openbanking.atlassian.net/wiki/spaces/DZ/pages/16320...

The biggest UK banks have a legal obligation to participate and many smaller ones are participating as well.

I can see why a third party API gateway would still be useful internationally though.

It could provide a useful shared API for all the countries it covers. I also think there's a non-trivial certification process which would be onerous for a small startup.