I'm eagerly waiting for a proper Mac Mini teardown. On the other hand the devil could be in the detail or, in this case, the wording on the Apple Store:
Apple Store note on MacBook Air storage:
"Storage is built into the computer. If you think you may need more storage capacity in the future, consider upgrading at the time of purchase".
Apple Store note on Mac Mini storage:
"Mac mini storage is not user accessible. If you think you may need more storage capacity in the future, consider upgrading at the time of purchase".
It is very possible that the Mac Mini SSD is a standard PCIe nvme with an M.2 connector. Simply not easily accessible but replaceable if having the proper tools and instructions (even if an upgrade will void the warranty)!