This is a cool attack, but I'm not sure where you got "$14" from. The laser pointers were $18 for a pack of 3, but you need a separate laser driver like the $300 one the authors used, and probably an audio amplifier as well.
Particularly as this isn't exclusive to smart assistants. They found the same could be used against phones and tablets (as slightly higher power due to the lower sensitivity of the microphones).
Yes, but it's targeting the aforementioned software regardless of device. (Not that the HN title isn't terrible, I agree with thsowers on using the title from the actual paper.)
Stipulating that $14 might be off by as much as two orders of magnitude, cost remains an insignificant barrier for any adversary with access to a credit card.
Modulating a laser at audio frequencies is pretty trivial.
You can probably inject the signal straight from the headphone output of a smartphone into the right pad in the laser pointer with a decoupling resistor and inline capacitor.
...if you don't care about audio quality and longevity of the laser.
There are some videos of the attack at https://lightcommands.com/