Were they actually using anything specific from their PhD? The research they did was directly relevant to some part of the HFT application?
I'm just showing you how it's possible to do a PhD then get a good paying job at the end. This removed the worry of missing out on 5 years of salary.
Why do people want degree graduates? Because it shows they can finish things, because it shows they are smart? because it shows they have drive? Many reason are given for hiring grads.
Somebody with a PhD in comp sci would have pretty much proven they are good with very difficult problems and have the drive to see things through to completion.
What qualities do you think it takes to finish a PhD? Some employers seek out those qualities and a PhD is a good indication that you have those qualities.