So what I recoomend is put on a hd and hide it some where. Go check it in 15 years
but I still don't understand what supposedly makes it more reliable long-term as storage.
But the tape itself has a great reputation for being physically robust. There are fewer parts to break than in an HDD, too. If your tape drive dies, you can replace it and keep using the same media. That’d be like having hard drive platters you could swap into another HDD later on.
This is always the risk. Longevity testing is often done at high temperatures or other artificial means but cannot exactly simulate 30-50 years of storage. If something is important, it's best to use two different media, and check them over the years.