It's apples and oranges. This Moderna page may help:

https://www.modernatx.com/mrna-technology/science-and-fundamentals-mrna-technology

DNA encodes mRNA, which encodes protein. Moderna fits in at the mRNA stage. Paraphrasing from their website, they start with the desired protein sequence for a protein target of interest. They then design and synthesize the corresponding mRNA sequence that cells will recognize and translate into a protein (ribosomes of the cell read the mRNA code and build the protein). They can tinker with the exact mRNA sequence to optimize the mRNA’s physical properties, as well as those of the encoded protein. They also have some tricks to specifically deliver the mRNA to different types of cells. In this way, they can turn on the expression of a specific protein in the cell.

Moderna is not fixing the person's DNA and therefore is not fixing any mRNAs made from the person's own DNA. They are administering a specific mRNA for the cell to make into a specific protein. Potentially very useful for vaccine development, immuno-oncology, and other areas, especially if you only need the cell to make this protein over a short period of time: https://www.modernatx.com/pipeline. 

Apabetalone and other epigenetic therapies are VERY different. Moderna's mRNA therapies are specific and the mRNA's can encode normal, mutated or foreign proteins, depending on the design and purpose. However, epigenetic therapies modulate the cell's transcription of its own DNA encoded mRNAs. Importantly, manipulation of epigenetics causes changes in expression of MANY different mRNAs and is not going to specifically modulate a single target.

BDAZ