Using the builtin browser button element isn't without other things to then fix in the software either, "(getting anything to automagically work in UI is a pain still)".

Would it have been accessible though, sure. But I'm going to assume that the developer didn't spin a wheel and decide to add React to the project rather it was intentionally chosen for some reason. Similarly in their React button version they could have added ARIA attributes. The problem statement was never "is there a way it could have been made accessible" it's that there isn't going to be a fullproof automagical way to make it accessible without having to do additional work in the project be it styling the default button or changing it's handlers or adding accessibility to a custom button.

i.e. it's not a matter of technical exercise for how it could have been accessible it's being realistic about why it wasn't.