Is there something specific in that process that required WebUSB vs just normal USB? Sounds like phone makers could have done this since forever if they wanted to, what makes WebUSB particularly useful for this?

Native android apps can talk to regular USB devices, if granted the necessary permissions. But it's exposed through a Java api (and Kotlin I suppose, these days), which is fine, but it means you need to write your client logic twice. If you target the web, you can do it once.

(Yes, you could try to bulid some common interface, libusb-style, but I think you'll have a bad time with minor behavioural differences, especially around permissions. libusb itself does ostensibly support Android but there are several caveats: https://github.com/libusb/libusb/wiki/Android#does-libusb-su... )

Cross-platform compatibility comes to mind. WebUSB is available on macOS, Windows, and Android; a native Android app would pose a bootstrapping problem for a probably not insubstantial fraction of all potential users.