If the MOSFETs don't meet the specs on Infineon's data sheet, including rad hardness, then Infineon would be in breach of contract.
Is my reasoning correct?
German law differentiates between "open deficiencies" and "hidden deficiencies". If you neglected to properly check for an open one, that's on you. You now have no warranty under the law. In case of a hidden one, which will likely only show during large-scale production and can't really be detected beforehand, you have to immediately report it once you discover it, and it is your responsibility to document & prove that you did so without delay.
Under this system it's up to the buyer to decide how much reliability they need. They can forego testing and save money because it's not important to test every single screw when building a garden shed, or they can rigorously test every single thing because they're building a spacecraft.
* It is enough to prove that you did perform checks. If you got unlucky and the random samples just happened to be good, you are still protected. But if you didn't check at all or not sufficiently, you're screwed.