Hahaha - you seem quite happy refuting a point.
Sorry for getting back to you late - that's a really cool video - but as i mentioned, the main point, in regards to cost, was that when compared to something that is ready made it's cheaper.
Also, while the video you shared is pretty awesome and somewhat straight forward, it's not exactly great for those with a tight schedule. Plus, while you can get a variety of textures, it would be difficult to replicate a ruined city or flagstones in a DIY if you didn't either make a stamp of some variety, or sit there sculpting the mat as it dried.
For me, and i'm sure many others, it's about making the most of what meager time you have for the hobby, especially when you work full time, study, and have a family.
Don't get me wrong, your method would be fantastic if I had the few hours to put into sourcing and prepping the materials and doing it myself.
I just mainly wanted to share how chuffed i was with the mat, which i thought was a good deal