No, the model is useful without the dataset, but its not functionally "open source", because while you can tune it if you have the training code, you can't replicate it or, more important, train it from scratch with a modified, but not completely new, dataset. (And, also, understanding the existing training data helps understand how to structure data to train that particular model, whether its with a new or modified data set from scratch, or for finetuning.)

At least, that's my understanding.