It looks to me like a somewhat unique opportunity to risk it because without this treatment, she was dead anyway and in fairly short order. So it was a means for them to learn something with very little potential downside. Worst case scenario: The treatment kills her. And she was dying anyway.

From the article:

In Alyssa's therapy, each of the base edits involved breaking a section of genetic code so it no longer worked. But there are more nuanced applications where instead of switching an instruction off you can fix a defective one. Sickle-cell anaemia, for example, is caused by just one base change that could be corrected.

And I already quoted (in my previous comment) the next paragraph that indicates they are already doing trials. This is potentially a big deal and they seem to be pursuing it in a very reasonable and conservative fashion, which is always something that concerns me with this type thing. There's a lot of potential for terrible and horrifying abuse and that is not what I am seeing described here.