There are software configurations that are easily parallelized. Connect the computers together with 10gb ethernet, and the software can distribute it across the computers.
It takes special software, it isn't something automatically done.
Why would you use 10gb ethernet, when you have 40gb thunderbolt 3?
I'm assuming what they were referring to in the presentation is daisy-chaining the USB-C/TB3 ports on multiple Minis and using IP over Thunderbolt networking.
XCode Server is a good example of applications that can make use of this. An XCode build can make use of networked XCode Server instances to farm out parts of an XCode build to other machines. Not specific to daisy-chaining or Thunderbolt. It just works over IP. But clearly 10Gb will do it more efficiently than 1Gb, and 40Gb will do better still...
I dunno what other applications might benefit. I suppose some video rendering applications, but if so you've picked the wrong device, as the GPU on the mini - while much better than earlier models - isn't suited to the purpose.
I had to choose between the new Mac Mini and an iMac Pro for iOS/Android software development. I've been using a 2012 Mini i7, and after retrofitting a "fast" 1GB flash SATA drive (OCZ Vector 180, 400MB/sec) I've stretched it to it's limits, and will probably not get the next major OS upgrade.
The new Mini delivered about what I'd expected (which is more than most here seem to have expected), but the iMac Pro is a better value, though more costly. For 50% more (I priced a high-end Mini configuration) you get nearly (multi-core) double the CPU performance, similar SSD performance, and a 5K IPS display thrown in for free.