n8w There are 37 US banks on that list, while there are 4600 FDIC insured banks in the US ("insured institutions"), according to the FDIC website. That's less than 1% of banking apps covered, assuming all of those banks have an app.
The banking app compatibility with GrapheneOS list is a crowd-sourced 3rd party community driven project. Those who have heard about it and wished to contribute to the list have done so. I'd suspect that many never would either; they simply just tried their banking app and it worked. I'd imagine that majority of the GrapheneOS user base has never even heard of this list.
Note: GrapheneOS does not make any guarantees regarding the list's validity.
I use a local credit union which is not on that list. I don't expect to find any information about it here since it's relatively obscure.
If you happen to try it some day and everything works out, please consider sharing your report by submitting it here. Perhaps one day, it might benefit someone else.
Additionally, there is always the web versions/PWA's and/or a banks regular website that can be accessed via the Vanadium browser.