Unlike the old rack mount options for the trashcan Mac Pro units (eg https://photos.imgix.com/racking-mac-pros), it doesn't seem like much thought has been given here to a front-to-back airflow.

I'm also surprised they're touting the density of this solution— seems like the obvious thing would be to put the Minis on their sides. A 4U chassis has 17.5cm vertical space in it, and a Mac Mini is 17cm wide. With the Mini being 2in in height, that suggests 8-9 Minis in a 4U rack, vs 2 Minis in a 2U rack for Scaleway's trays.

EDIT: Here's a commercially-available solution that's 6/4U: https://www.mk1manufacturing.com/Rack-Mount-for-6-M4-mac-min..., you'd think it could basically be this but with the management plane behind or in front. And as others have said, making the management plane be more shared, so it's not 1:1.