I am not saying that this is low effort either, the website itself probably took some time. I am saying that a lot of this effort is pointless.
To be clear, not because the developer is bad (I did not try it outside of the website demo), but because of the state of software development imposing pointless complexity.
How hard do you believe it should be to play a sound on keystroke?
The devil is in the details.
So I am a bit careful making a statement here without trying to implement it myself first. Could be super simple, but maybe not.
In an ideal world, this app should be programmable in the same time it takes to read the website and buy it.
Not questioning the reasons why such app need to be paid seems even stranger to me.