For other advanced stuff you have to be the default dialer app which also makes a lot of sense. If you're going to be an accessible dialer, you may as well actually communicate to the system that that is what you are. With plenty of open source dialer apps to borrow from, I don't expect the burden to even be that high.
Interestingly, the post writes that the app communicates with the phone over ADB. I wonder if you could just inject keypad button presses using adb, so the app doesn't need to even be an accessibility service to get the tones across? Although I guess that only works if the dialer is in the foreground.
No, though I will say I'm upset with how protected calls have become on phones in recent years. I used to record every phone call with an app so I had a record of it, something which is completely legal to do where I live, but that really isn't possible anymore.
Now all my calls go through a VOIP provider and call recording is a feature they offer.
So a trrs plug splitter I can record to a tascam. And use a fancy mic.
Still, nicer to press a big record button, by a lot.
But AFAIK both Android and iOS disabled all call apps except for their own.
Previously they allowed as long as the keypad looked "distinctively different from the default call app", but turned that off some time ago. I can see the security reasoning, but still think there were ways to make it possible and secure.
A similar question, for example, is how to record a call, default or not. AFAIK there's no way.