Skainet does work, but the accuracy is insufficient to pass Alexa qualification. Espressif is aware of that and they are working on alternatives., Meanwhile if you want certified Alexa on ESP you have to use external wakeword chip from one of the vendors with Alexa certification. Alexa certification is quite hard, you are only allowed to make three errors in 24 hours of testing.
I have no confirmation from Espressif, but Tensilicia does offer an AI coprocessor that could be integrated into a future ESP chip.