That download said to have the latest sigpatches. Before downloading that one I tried getting sigpatches from the official site and it CLAIMED to support 18.0.0, but it didn't work for me.
Oh your talking about hacking method? Well I just installed the NSP, like I do with any other NSPs and it works without and issue. (There was no homebrew or hacking on the category tags, so I got confused, sorry)
Did you check the eshop? Go to the redownload section of the eshop, and check if Splatoon 3 is there. I had the issue as well, so I checked the redownload section of the eshop, and I saw Splatoon 3. I clicked Splatoon 3 to redownload and it...