He's way lowballing some of his assumptions.
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For example, he lists the Radeon RX580 as $200, but the Mac Pro has a Radeon RX580Pro with special Mac-specific firmware, which I cannot find for less than $350. Also, the version Apple is using is passively cooled, which nobody sells out of the box. But you can buy a passively cooled backplate for it, which runs an additional $150.
So the video card alone he assumes costs $200, but it would actually cost $500.
As another example, there is no way a generic EATX chassis would come close to comparing to the Mac Pro chassis, and its cooling system. Apple says the new Mac Pro runs at less than 10db, how much is that worth to a buyer? I don't know of any comparable cases, but other top of the line well-engineered cases easily cost $500+. Same with the PSU - I highly doubt a $300 PSU will be as quiet as Apple's. Passively-cooled high-watt PSUs are very expensive.
you can buy any retail AMD Radeon RX 580X 8GB for €180 and plug it in a macpro and it will work.
http://www.xlr8yourmac.com/archives/feb18/RadeonRX-580_MacPro.html
That AMD Radeon RX 580X will outperform the Pro 580X, it is the same chipset as the Radeon RX 580X but with slightly higher clock speed.
https://www.techpowerup.com/gpu-specs/radeon-pro-580x.c3398