A 10 Gbps network is MUCH slower than even the smallest oldest PCIe slot you have. So cramming the GPUs in any old slot that’ll fit is a much better option than distributing it over multiple PCs.

Technically it’s empty space that’s being wasted, if you fill it up it’s being useful!

Regarding the second point, militaries have a long history of using civilian products (see GPS in the gulf war for example) if they don’t have enough of the expensive stuff, which will lead to incentive to jam the civilian stuff as well. The article suggests that this system should be more resilient against simple jamming though.