It's not an advancement thing, its a cost thing. They made the filter good enough to block out the stuff people can't hear but this device is super loud so you would need additional layers of filtering to completely block it. Smartphone mics need to be tiny so additional analog filtering is going to take up space and resources. You could also run a higher sample rate on the ADC that converts the sound to a digital signal and run a digital filter to cut off the ultrasonic band but that requires more power and chip resources and the ADC might need to be swapped for one capable of the higher sample rate. The tools are all there to defeat this but it's a matter of reducing power, cost, and computing resources.
Perhaps, though not easily. You could be looking at getting harmonics on the physical components of the microphone from the ultrasound which would bleed all the way through the normal sound ranges. Plus, with enough sound pressure, the microphone may be physically maxed out (that is, physically/electronically unable to register any further sound pressure).
Seems like countering it might be a use-case only important to specialty buyers who want to get around this kind of protection. Even if it wasn't that difficult to counter, there might not be enough incentive for most off-the-shelf mics and specifically Alexa/phone/etc to do so.
