First, we don't know if the maker's soap is fully saponified when it's cut and placed on a rack to cure. It might be mostly done, but even that last 1-2% of saponification will react with aluminum and cause it to deteriorate. I can show you one of my aluminum cookie sheets as proof that freshly made soap can deteriorate aluminum. (Soap that I thought was fully saponified, but obviously wasn't)
Second, it takes weeks, even months, for soap to cure, so soap on metal racks is exposed to that metal for a long time. Soap is fairly sensitive to going rancid from exposure to metals and the longer the soap is exposed, the greater the chance to develop rancidity. Some metals such as iron and copper are especially bad. Even if the rack is coated with paint or other covering, metal ions can migrate through these coatings and trigger rancidity in the soap.
You're going to be using soap dough that's older so it's more likely to be fully saponified. Also the soap dough is exposed to the aluminum tool for only a short time -- minutes to hours, not weeks to months. Your situation is low risk compared to curing soap on metal racks.