Never.
Wrong hammer for the job - we agree on that. But nothing I saw on their online pages expected that the Mac Mini would ever stand up to the ultra high density server and storage solutions that exist now.
I thought the discussion was directed at small business security, not convincing Facebook to buy pallets of Mac Minis to stuff in their new Fort Worth data center.
I will say - with the proper software and ingenuity, a fanless SSD box like the Mac Mini could be ganged together to create such a monstrosity.
After all, even Saddam used PS2 units for missle or nuclear design.
At this point we don't know what the scale is, or what the original infection vector was. I do know that "replace it with Apple" is not a viable solution for anything beyond a home or small business environment. They simply do not have anything that will scale beyond that. Ransomware encrypts your data and then you have to pay to get the key to decrypt it. It can do this to any file the user has write access to, so it doesn't need admin rights to do a lot of damage. The target vulnerability is the user, not the OS.