If she's wrong it's not the end of the world. You don't need 100% accuracy for the solution to be useful. In fact you probably need to attend to whatever else was beeping anyways.
1. You tell Alexa to let you know when the oven is preheated.
2. She hears the washing machine beep and tells you the oven is preheated.
3. You come back, find that the oven is still not ready.
4. You conclude it's probably the washing machine and go check it out, forgetting to tell Alexa to let you know when the oven is preheated.
It's kind of like saying that using a rotary phone UI on your smartphone would have been okay, because we've managed with it just fine for a long time back when we had rotary phones.
Perfect is the enemy of good.
The point is that the audio pitches aren't outside human detection and don't have distinct enough patterns right now. Changing pitch and patterns would require appliance manufacturers' support, at which point you might as well go with mature, well-defined and well-implemented solutions, such as WiFi connectivity, rather than defining and implementing your own protocol based on audio pitches and patterns.
It's just a thought, like I'm against the whole thing in general, but it seems like this is doable without needing a lot of support
Also despite sounding similar to tone deaf humans one probably runs at 4001.342 Hz and the other at 3999.114 Hz which likely sounds the same to me, but to a computer is likely very obvious.