There is a huge issue with this inference drawn from experiments in the paper. The authors use OT-1 mouse. This mouse a T cells expressing an artificial T cell receptor specifically designed to recognize Ovalbumin peptide from amino acid 257-264. The mice is helpful in studying T cell biology. However, if you change the peptide sequence itself, it is bound to have an effect on T cell activation. How much of activation is lost will have depends on the nature of mutation you do to the peptide. So it doesn't matter whether you use terrestrial or extra-terrestrial amino acid. If it changes the structure of the peptide, it will reduce the amount of T cell activation. Or in few cases even increase it. But there is a catch. Since mice T cells was designed to identify that specific sequence, it is already near maxima of activation when they see this peptide, So any change will either give you same amount of activation or a reduction in it.
Think of it this way. You are used to Qwerty keyboard, You have a certain speed and efficiency at typing. It doesn't really matter if I rearrange a few alphabets or replace some of them with Chinese/Hindi alphabets. It is going to decrease your effect your performance in typing. Hence, changing the keys doesn't tell you anything about your ability to learn Chinese. All it tells you is that you were artificially trained on qwerty but now you don't have a qwerty keyboard.
Similarly, this study tells you nothing about body's ability to react to alien amino acid. All it says it T cells were designed to recognize specific sequence, and that sequence was changed. But that's what author did intentionally anyway. So what is even the point of this obvious study?