My 2018 Model X has terrible battery, roughly 120 miles ;(

[MP3Mike]

Perhaps I'm mistaken but I've read that 70% retention rate in my 8 year unlimited mile battery warranty. Have you or someone you know confirmed that Tesla only covers failure of battery?

There is no warranty document that has 8-year/unlimited mileage and a 70% degradation coverage. The warranty that came with your vehicle makes it very clear, and the current warranty document refers you to it:

I don't have an old copy of the warranty handy, but you should have it linked in your Tesla account.