80GHz doesn't leak in every direction the way 5GHz does. The antennas have 9 channels (each 500MHz wide). Also, you can reuse the 9 channels by using a horizontal polarity instead of a vertical polarity.

Well, there's 10 GHz of available in this band (71-76 GHz and 81-86 GHz) and full-rate gigabit can be achieved typically with 750-1500 MHz of spectrum (depending on vendor and, mostly, modulation).

In addition, this spectrum requires a license (as low as $75, IIRC) and they are pretty easy to get in this band ("lightweight licensing").

Also, on top of that, an open network with no encryption? It seems like that would be a great place to do some MITM attacks, as well as a variety of other things

These are point-to-point links so it's not "shared" bandwidth as with, for example, cable modems. It will be dedicated links between the endpoints.