This was a surprise: Severance is edited on a Mini amongst other machines like MacBook Pro. It's in the "Behind the Mac" video linked in this page. They must have some large external Thunderbolt disks or now use 10Gbps networked arrays,

Mac - Lumon Terminal Pro
See how Mac — and not the Lumon Terminal Pro — was used to edit the Apple Original series Severance.
www.apple.com