When talking about beta thalassemia we must always remember that many different mutations of the beta globin gene are all called thalassemia even though these mutations have very different local effects within the chromosome.
There are various types of beta globin defects and mutations that affect the production of beta globin. Some are outright deletions of the gene, as with beta zero. These would not be helped by correcting the MRNA messaging. However, many mutations are of a variety where the gene is basically malformed, disrupting the normal messaging path of MRNA. The result is that the command to produce beta globin does not follow the correct path. It is almost like a car going down a one way dead end street. The goal of correcting MRNA messaging is to get it to direct the command down the right street.