There are, it has to be said, cheaper ways to build a server. If you really must test a lot of Apple hardware at once though, it's at least feasible.
The interesting thing about this particular hack, cost and Mac fanaticism/hatred notwithstanding, is how it's been managed. 96 MacBook Pros are going to pump out a lot of heat, after all.
To combat that, three Macbook Pros sit in each bay of a server rack, separated by 3D printed spacers to keep the lids open 7mm and give them separation for cooling purposes. Exhaust fans pull the hot air away from both the MacBook Pros and their power bricks, and finally thermocouples monitor the temperature of each MacBook unit individually.