Native is iirc 30TB - they quote compressed capacity but eh that's very much going to depend on what you are storing and how compressible it is.

And you'll have a rough idea what it is you are going to be storing and how compressible it is if you spending that kind of money.

It's marketing and a little skeezy to quote it and I bet they have some justification for why they arrived at 2.5:1 compression.

EDIT: Yeah it's 30TB - been many years since I had anything to do with LTO but they use a modified version of LZS called SLDC so it's that that they are assuming will get 2.5:1 on "random enteprise data that isn't already compressed" the 2.5 threw me as well because that used to be 2:1 so either they improved SLDC or thought they could wing it - looks like that switched between LTO-5 and LTO-6.