It contradicts your statement: "it's not as simple as having someone at Hospital X download a pretrained model in Tensorflow and train its last few layers on some CT scans" Because in this case it was as easy as taking a model from a totally different modality and retraining the first (in this case) few layers to accommodate the new device. Furthermore the original training used 15k scans and the retraining only required 152 scans. This is totally reasonable and clear evidence of transfer and generalization. Moreover, even human operators require retraining on new devices!