I think you're misunderstanding the example use-case (it's understandable, it took me a decent amount of completely focused time to really understand zero knowledge cryptography).
The use-case with medical data described isn't like homomorphic encryption (where computation is done on an untrusted device, but with encrypted inputs/outputs). It would be more like: being able to prove you have some medical condition to an insurer, without your doctor having to provide them the lab results, but instead providing a ZK proof of an ML model that detects that condition, for example.
Using these kinds of proofs, you can start to silo your information off to just the parties that absolutely must have access to it, while also allowing parties that need to be able to verify certain narrow attributes (like an insurer needing proof you needed some procedure) to verify it without needing to invade your privacy.
The use-case with medical data described isn't like homomorphic encryption (where computation is done on an untrusted device, but with encrypted inputs/outputs). It would be more like: being able to prove you have some medical condition to an insurer, without your doctor having to provide them the lab results, but instead providing a ZK proof of an ML model that detects that condition, for example.
Using these kinds of proofs, you can start to silo your information off to just the parties that absolutely must have access to it, while also allowing parties that need to be able to verify certain narrow attributes (like an insurer needing proof you needed some procedure) to verify it without needing to invade your privacy.