Halifax is owned by Lloyds banking group, and their current app is just the exact same Lloyds app with a different logo. I know because I bank with both and the apps are identical.

Previous to the merging of online services, you are correct that Halifax had its own app and it was terrible. But at that time Lloyds had a great app, they just hadnt unified the back end tech of all the different bank brands they own.

It wasnt disruption from startups that caused the improvement, it was the parent company taking its time to merge the decent tech it had developed for itself.