Tether didn't prove anything other than they have $1.8 billion in an account. There is no guarantee from the bank that the money will stay in the account for any length of time or where it came from. It could be a short term loan for all we know. There is also no guarantee that the money is not encumbered by some other financial derivative.

In short, it is an interesting data point, but it is far from proof. The only thing that would truly prove the money is there to back Tether is a complete audit, and that is something Tether is still steadfastly refusing.  Most people are still wondering why, bank letter or no bank letter. This will likely change the narrative though, and may stop the bleeding for a short while.

Don't expect it to do anything about the rest of the crypto market though. I'm not even sure it will help Tether. It is still too sketchy.