The Legacy Pro product that you have is likely the same, or very similar to, the Kodak F-6 formula (or maybe F-5 or F-7). In any case it is the alum (usually potassium aluminum sulfate) in the mix that does the hardening. This was industry "standard" for decades before rapid fixers became common and emulsions were pre-hardened.
This is also a "conventional" sodium thiosulfate fixer, so fixing times will be in the 5-10 minute range. Since it's hardening, washing times will be slightly longer. For fiber-base papers, I'd try to use a non-hardening fixer, but you can use this too, just make sure you use a wash aid and wash extra long.
Doremus