I don't know how they managed to get the outside of the chicken so crunchy:
To get a crispier batter, you need to experiment with different starches. Most people will use wheat flour to make their batter. This gives you a nice soft crispiness, but it has a few downsides - it goes soggy easily - steam escaping from the chicken will make it soggy. For this reason it can not be doused in sauce and expected to maintain crispiness.
For some time now, chefs like Ferran Adria have used Dextran based starch like
Trisol. Mix 30% Trisol with 70% wheat flour and proceed as normal. This produces an incredibly crispy result which retains its crispiness despite being doused in sauce. The problem with Trisol is that it is difficult to source it in small quantities. For this reason, I have a bucket of Trisol that I have been slowly using for the past few years.
Asian cooks mix wheat flour with cornstarch, or wheat flour with rice flour to produce different types of crispiness. In fact there is an emphasis on using different types of starches in Asian cooking that seems to be missing in the West. Japanese
Agedashi Tofu is a deep fried tofu dish which uses a mixture of wheat flour and cornstarch or potato starch in the batter. It is then served with dashi broth. If you were to use 100% wheat flour, the fried batter would fall off the tofu as soon as you immerse it. Adding cornstarch helps the batter stick to the tofu as well as give a sort of glutinous texture to the batter when it is wetted by the broth.
Maybe we could have a "Food Science Review" on ASR