Spyware using ADB could be a thing (assuming someone has full access to your phone, including the password). However, it is impractical, as a reboot, system update, or Wi-Fi change would likely break everything.
It is much easier for spyware to be installed as a device administrator, granted accessibility privileges, and then given every possible Android permission via AppOps as a one-time setup.
At that point, ADB no longer matters. The spyware is already configured and ready to run. Even if on-device ADB were patched, anyone with physical access to the phone would still be able to install and configure their spyware via a USB cable.
As for attacks without physical access, as explained in the article with the 3 scenario, this is neither practical nor realistic. It only affects a very small subset of users (primarily Android developers) under very specific circumstances and during limited time windows.