Just installed a 3rd party app on my pixel 3, made it the default and tried this. Worked just fine and popped up the 3rd party app.

But why bring up Android at all? Halide doesn't work on Android and this entire discussion is about iOS.

I can bind double-tap on the side key to any app, and my lock screen launcher can have any complication it likes. So, yes?

That's not true. You can change the lockscreen shortcuts on Samsung phones for example