You are comparing different things. Sure, if Apple made a dual-processor machine with room for 4 GPU's...
...but they don't.
The correct comparison would be between the single-CPU 12-core MacPro and an equivalent current Xeon 12-core (I believe Haswell is the current gen? or is it Broadwell?). The GPU situation is different and harder to compare, and it has been discussed ad nauseam here.
Car models are typically around for five to six years. Cosmetic mid-cycle refreshes have become more common, but introducing new models takes about five to six years (production lead times simply take that long). The US custom of making minor changes to differentiate between model years of the same car is a marketing tactic, the car itself remains the same. And prices remain constant.
Well the only reason why that guy on the video didnt compare it with the 12 core Mac Pro is because he wanted to at least try to keep the prices comparable. And, that should be an important factor. THat for the price of a 6 core Mac Pro you can get a pc workstation that performs a lot faster.
But, ok lets compare apples with apples then. I bought a PC for about half the price of the compared Mac Pro from the video. It has 6 cores of Broadwell-E - so the same amount of cores and threads as the Mac Pro. My photoshop benchmar took 9 seconds, so thats about 2/3 of the time it took the Mac Pro machine that cost TWICE as much. In addition I have a GPU that performs 5x as as good as the Mac Pro. THeres really no point in arguing that well, the Mac Pro has Dual GPU, because those dual GPUs are rarely used in any programs and even if they were they would perform less than a GTX 1080. Then ram, I have twice as much ram as what you can actually get on the Mac Pro which in addition is DDR 4 and 1000mhz faster....and I could keep going...
So, yes I understand he was comparing two different kind of machines, but he was trying to make a point from a pricepoint perspective. And even if he bought the ridiculous 1500$ more expensive 12 core, the PC would still run circles around the Mac Pro.
So no matter how you compare, from price, hardware or any kind of way, it simply doesnt justify buying the Mac Pro. And honestly, cars yes they make cosmetic changes in between big cycles, but thats an industry that has been around a lot longer with more incremental changes...and dare I say, more complex constructional costs. And apple doesnt even make their own internals, the hardware is available, they just like to sell you the old stuff...I honestly find it peculiar, how they still get those parts. I would assume for Intel to deliver apple 3 year old Xeons would be more costly than to give them new ones, since they are mass producing new models.