I have an indoor rabbit and wanted to build a nice digging-box for it. Is this soil safe to use?
Looking at the review for that particular product I find that the product varies by location, there is little if any quality control. It is not possible to say if it is safe or not, even if you purchase a bag and it is good, the next time it may not be. It does not appear to have any additives in it, which is good. Many potting soil products have additives that are unsafe. In the end unless you test each lot, you can't know what is in the bag
You are 100% correct that a digging box will improve the quality of life for your pet rabbit. The question My rabbit is digging at the carpet in the corner of the living room, what should I do? includes some other benefits and includes an answer with some pictures of different digging box designs.
Your rabbit is going to use the digging box as a litter box, and dirt is not an optimal substrate for indoor litter boxes. I recommend using the same litter in the digging box that you use in the litter box, or failing that use one of the pellet types, see related What do I need to consider when buying litter for a rabbit?
Store bought soils are not going to be a good choice, for a digging box. I recommend using about 5 quarts of litter in the bottom. I change ours every other day, with 2-3 rabbits using it.